You could also vary the number of supplies required based on the prevailing conditions, to simulate environmental challenges. For example, ovbiously a rest in any settlement probably doesn't use any supplies, while a long rest in a harsh environment might use three or even four unless they scrimp.
I was thinking in these cases to just impose disadvantage on gathering checks. Sure, there are environments where you can't find any food (or what forms that food takes can be...
unique), but I'll burn that bridge when I get there. Supplies being more than just food/water has to be taken into account as well- you might not find anything edible, but you might find
something!
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Second Revision!
Supplies
Instead of tracking things like inexpensive material components, ammunition, or rations, this campaign will use the following rules.
Short Rest: characters need to have one unit of Supplies to take a Short Rest. The idea is that you're maybe having a quick meal or snack while you rest. A Short Rest takes 10 minutes to complete. While taking a Short Rest, characters can Use Objects, Hide, Search, or possibly use some Class Features. Other Actions may be taken with DM approval. A character can take only 2 Short Rests per day.
Long Rest: characters must consume two units of Supplies to take a Long Rest. During a Long Rest, you can take most Actions, but are unable to cast leveled Spells.
Interruptions: being interrupted does not necessarily make you unable to complete a Rest. Normally, if you are forced to take Action or damage that would prevent a Rest, the time needed to complete that Rest is simply increased by 5 minutes for a Short Rest or 1 hour for a Long Rest. If, however, the interruption is longer than 10 minutes (Short Rest) or 2 hours (Long Rest), you must start the Rest all over again.
Gathering Supplies. Supplies can be scrounged from the environment with a Wisdom (Survival) check; success grants the character one unit of Supplies. This requires 1 hour of time per check. The base DC is 10, though the player may have advantage or disadvantage depending on how plentiful the area. The DC goes up by 1 for each roll made in the same area.
Note: the Outlander Background allows you to find an additional unit of Supplies, even if you fail a roll, but no more than 5 bonus Supplies can be found each day. The spell, Create or Destroy Water, does not create a unit of Supplies, but does provide advantage on checks to forage for up to 10 checks.
Note: the spell, Goodberry, in addition to it's healing benefits, counts as 1 unit of Supplies when consumed. Create Food and Water, creates 30 units of Supplies.
There is a base 1 in 6 chance that gathering Supplies in an area might uncover something special, be it rare herbs, gourmet mushrooms, feathers that can be made into special fletching, or exotic spell components. Check once the first time Supplies are gathered in an area; no matter how many checks are made, no new finds can be discovered at this time.
Other ability checks may need to be made to harvest special materials, at the DM's discretion (Arcana, proficiency in an Herbalist or Poisoner's Kit, and so on).
Some areas have a higher chance of locating special materials, or have greater quantities of them available!
Note: the Natural Explorer ability of the Ranger increases the base chance to make a discovery by 1 in 6.
Purchasing Supplies: a unit of Supplies costs 5 sp and weighs 2 pounds. Exceptional Supplies can be purchased as well. These cost 5 gp. A unit of Exceptional Supplies grants the character a +1 bonus on weapon and spell attack rolls until they take their next Long or Short Rest. When taking a Long Rest using Exceptional Supplies, the character gains the ability to take a third Short Rest on the following day.
Mixing Supplies: a Long Rest taken using a unit of Exceptional Supplies and standard Supplies can grant the attack bonus or the bonus Short Rest, not both (the character can choose).
Special Supplies: Dwarven Mine Rations, Elven Waybread, Halfling Sling Stones, Firebat Guano, and the like, all exist, which can grant additional properties, but cannot be purchased just anywhere.
Cooking: while Supplies consist of more than just food and drink, most species can benefit from properly prepared meals. An Intelligence check can be made before using a unit of Supplies- if the character has proficiency in Cooking Utensils, they can add their proficiency bonus to this check. With a check of less than 5, the Supplies are ruined and cannot be consumed. With a check result of 10, the first healing surge spent during this Rest adds d4 to their result. If this check is exceeded by 5, for every 5 points, add another d4 (thus a DC 30 check would provide a meal with miraculous healing benefits, adding 5d4 to the first healing surge spent!).
Note on Species
Some species require less food or drink, or don't eat or drink at all. They still require Supplies for things such as ammo or non-costly spell components, but obviously, encounter much less hassle when acquiring them.
If you require reduced water, add 1d4 to rolls made to forage for Supplies. If you require no water, gain advantage on rolls made to forage for such.
If you require reduced food, you further only need one unit of Supplies to take a Long Rest. If you require no food, there is a 50% chance when you take a Long Rest that you gain the benefits but retain the Supplies.
Running Out Of Supplies-
In the course of events, characters may find themselves with no Supplies. If this occurs, they cannot take Rests. Characters cannot regain healing surges or recover from Exhaustion, and each day spent without a Long Rest gives them another level of Exhaustion.
Penalties
In addition, going days without consuming any Supplies begins to erode character's ability to function in other ways. Each day, the party is subject to a Strife level.
Strife level 1: Ammunition and reserves of spell components begin to run dry. The first day this occurs imposes a -1 penalty on weapon and spell attack rolls.
Strife level 2: characters suffer Disadvantage when attempting to use weapons with the Ammunition property (replacing the -1 penalty), and attempts to cast spells with a non-costly Material component have a 1 in 4 chance of failure.
Strife level 3: weapons with the Ammunition property cannot be used at all, and attempts to cast spells with a non-costly Material component have a 50% failure chance.
Strife level 4: attacks with melee weapons are made with disadvantage (replacing the -1 penalty) and spells with non-costly Material components cannot be cast at all.
Recovering from Strife
A Short Rest recovers one level of Strife. A Long Rest recovers up to 2 levels of Strife.
