D&D 5E Out of the Abyss: number of encounters per day in the Underdark?

[Jester David]

Do those five encounters advance the overall story? Or are they just just five additional encounters for the purposes of having a full adventuring day?

The former is good. The latter is just padding, which is detrimental to the actual story (needlessly expanding the campaign while adding nothing).
If they're just there but don't actually confer any really challenge (i.e. there's no chance of death, no interesting interactions to be had) you could just narrate them. "You fight your way past a few additional swarms of spiders, but dispatch them as easily as the last group."

Really, random encounters are good for a change of pace at the table. After a bunch of roleplaying and travel, a random encounter is good to give the action orientated PCs something to do. When the attention starts to lag and people start building dice towers, you throw out a random encounter. It doesn't matter of it's just one for that day and success is a foregone conclusion, because the fun of that encounter doesn't come from winning or surviving, but engaging in that encounter.
Adding two more in rapid succession (let alone four more) doesn't have the same "break" aspect. Because, closely after three combat encounter, you don't "need" a break from roleplaying and exploration.

 

[Flexor the Mighty!]

The wandering portions of OOTA are the weakest part of the adventure...and it is a horribly weak part. I don't think you have any conception yet how much wandering there is in that adventure. I didn't until I got to the later parts where the PCs are taking multiple 60 day long journeys! It is an absolute grind and basically unworkable. Not sure why they even bothered with the tables. If you used the tables as indicated, the PCs would probably encounter every encounter on it literally 20 or more times. Possibly close to a hundred or more times for the table in Chapter 2.

My advice, having run the scenario, is this:

Yep. That is so very true. And if you want to have the recommended encounters per day then you just make the wandering parts longer as you run though combat after combat of meaningless foes. For often months at a time as you and a massive group of NPC wander the Underdark.

 

[Tony Vargas]

In short: have any of you considered creating, say, five encounters and then having the party deal with them (fight them, avoid them, etc) all in the same day?

I'd probably just deny long rests until the end of the journey, with nights on the journey counting as short rests, instead.

 

[Stormdale]

I'd probably just deny long rests until the end of the journey, with nights on the journey counting as short rests, instead.

Sam. these days I tend to turn wilderness/exploration section into a 4-5 room dungeon, which is how I prefer to do my wilderness exploration these days and gloss over the mapping and day to day stuff.

I tend to preplan them and would do something along the lines of:
• Combat encounter
• Role playing encounter- may be a party of svirfneblin miners who have some useful info about a hazard up ahead.
• Underdark obstacle/hazard to overcome- eg a chasm, bridge, dangerous terrain
• Trick/trap/puzzle encounter
• Possible plot twist/complication to storyline or other event

Or even just adapt the 5 room dungeon to a small dungeon they find on the way and run that as their travel encounter on the journey.

Stormdale