I worry about your choice of fish ... If you keep guppy, I suggest you to keep ONLY guppies!! Their fins are usually long, and attractive ... very easy for other fishes to nip. In the end, you may end up with injured or sick guppies with torn fins. Quite unhealthy for the overall tank.

As for the number of fishes, many people say 1 inch of fish per gallon ... so 10 inches of fish or 10 pcs of 1 inch fish (length of adult fish). That is the rule of thumb. But really, it only depends on 2 factors ...

1. Filtration setup. If you have a HUGE filter, and change water every day, you could, in theory, fill up the tank with as many fishes as you can fit!!!

2. Territory. Cichlids are famous for being territorial. You might only fit 1 male + few females for a Cichlid tank. For peaceful fishes, you could fit in quite a lot of fishes ... if you've got a GREAT filter system.

But I'd suggest you to keep to the rule of thumb!!!