Inferno faction discussion.

Initially, there were 32 destroyers. Strange, but true. Now I'm playing with a level 3 gate – it summons units that are 1/4 and 1/3 the size of the base. Something like 40 sadists with a base of 130.

The number of summoned creatures does not depend on the stack from which they were summoned. It depends on the total amount of HP of your entire army at the beginning of the battle (i.e., the HP of all stacks in your army).
For example, you have 10 maniacs and 10 succubi. You also have a gate that gives 11% (i.e., it's maxed out). You choose 1 stack of succubi, and 2 succubi are summoned (although, according to you, only 1 succubus should be summoned). So, we get (23*10+20*10)*0.11=47.3. That is, you will have 47.3 HP allocated for summoning succubi. The HP of a succubus is 20, which means you can only summon 2 succubi.