General rule of thumb is one inch of fish per gallon of water. You just have to account for how big the fish can get, not how big it may be currently. You can go beyond that but it all depends on your filtration and it can increase your maintenance. There is a calculator on the site that can give you a good idea.
Same rule applies, but you obviously have some time before the ill-effects of over population are felt as the babies are growing. I have a 75g with 40 adult fish and over 50 babies. If all babies make it to adulthood then my tank is probably over popuated, but right now that is not the case. Plus, I know they will not all make it to adulthood for various reasons - mostly being they are in with the adult fish and surviving is up to them - they have more than adequate places to hide. If you want them to live, you'll need to add what I call breeder grass or some other form of hiding areas.
The females can produce young about every 3wks and how many they have largely has to do with how large the fish is. For example, I have a few pregnanat guppys in my tank right now. One is nearly 3" long and huge and ready to pop. The others are average sized and just fat. They will probably have 5-6, but the big one could have as many as 20.
In my opinion over population and what occurs from it is at your hands. Keep it under control and although you may be over populated by sheer numbers, you really aren't. I plan to go to over 100 adult fish in my 75g. This may require a step up in filtration and high % weekly water changes but if I'm prepared to deal with it then nobody can say I'm over populated if my nitrates are kept low.
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