First thing is tell the whole group that you think you at at or slightly beyond your limit of players for DMing. If the others haven't DMed, explain that prepping, controlling and most importantly responding in session becomes much harder as the group grows. Else, you may find the group growing again.
Yeah, this. First rule of holes: When in one, stop digging. Or in this case, stop your players from digging for you.
Now, to the mathematical challenge of adjusting monster stats, I would suggest the following:
When possible, simply increase the number of opponents to match the increased number of PCs, and don't concentrate fire too heavily. This is the easiest way to keep things in balance.
When that isn't feasible (e.g., the party is fighting a single monster which would not logically have minions), you'll need to adjust the monster's stats. My suggestion is to double its hit points and add 40% to its damage output.
<math geekiness follows>
The way I came up with those numbers was a calculation of "combat power," which is the product of average damage output per round times hit points. I find this is a very useful tool for coming up with ballpark estimates of monster effectiveness.
Let's say you've got 4 PCs, each with 10 hit points and dealing 1 point of damage a round. How tough does a solo monster have to be in order to be an even fight? Well, for simplicity, we'll say the monster hits for 10 points of damage a round--killing one PC every round. The monster will take 4 points of damage the first round, 3 the second, 2 the third, and 1 the fourth; so if the monster has 10 hit points, it will die at the same moment the last PC drops.
This means the monster must have 10 times the combat power of a PC (10x damage, same number of hit points). You'll find that any combination of hit points and damage output resulting in that ratio will lead to the same outcome. The monster could have 20 hit points and deal 5 damage a round (5x damage, 2x hit points), or 50 hit points and 2 damage a round (2x damage, 5x hit points). The fight will last longer, but it will end the same way.
Now, run the same calculation for 7 PCs. Turns out, the monster dealing 10 damage/round now needs 28 hit points in order to be an even match. So the monster must have 28 times the combat power of a PC.
Hence, if you're transitioning from 4 PCs to 7, you need to boost the monster's combat power by a factor of 2.8. You can do this in any number of ways. If you emphasize increasing its hit points, the fight will get longer and grindier; if you emphasize increasing damage, the fight will get swingier and more lethal. I think 2x hit points and 1.4x damage strikes about the right balance, but YMMV.
Obviously, this doesn't come close to reflecting the full complexity of monster and player combat options. Be especially careful with monsters that inflict status effects, have save-or-die attacks, and so forth. Still, as a rule of thumb, the combat power calculation is one I've found fairly useful.
(I am assuming here that you don't mind making quick-fix adjustments to monster stats. If it's important to you to follow the monster creation rules to the letter, you're going to have a lot more work on your hands.)
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