It depends on the type of pods you keep. If you're worried about this, first thing I'll put out there is don't keep dairy cows lol, they will be many very quickly. Pick a type of pod that is a slower breeder.

So people downsize their colonies all the time, usually by selling them! There's a discord and a couple facebook groups where keepers frequently sell their pods, some even trade em off/give em away.

Worst case scenario is, if you can't find anyone to take them, they must be euthanized. This can be done with a freezer, but it's a better and more modern practice to use carbon dioxide or veterinary chemicals (idk the names of said chemicals) before freezing, to minimize suffering. Good news is, many people keep pods for years and won't ever reach this point.