The number, and why you start under it
The comfortable working range for a 20-gallon tank sits in the low teens of adult fish — but guppies are livebearers, and a mixed-sex group rewrites its own stocking level every month or so. Females give birth to batches of fry on a roughly monthly cycle, and even with the adults eating some of them, a colony that starts at ten can be thirty before you notice it happening. So the real advice is to stock for the colony you will have: begin with six to nine adults and let births carry the tank up to its working population, rather than buying fifteen on day one and spending the year rehoming fish.
Sex ratios decide the tank's mood
A sensible start is two or three males with about twice as many females — roughly one male for every two or three females. Males court constantly, and spreading that attention across several females keeps any one female from being harassed into hiding and exhaustion. All-male groups are the alternative when you want the colour without the colony: twelve to fifteen males fill a 20-gallon handsomely and no fry ever arrive. What does not work is a single pair, or one male with one female, where the female gets no rest from his attention at all.
Plants are your population control
How many fry survive is largely a planting decision. Dense stem plants, floating plants with trailing roots and clumps of moss give newborn fry the cover they need to escape hungry adults; a sparsely planted tank lets the adults do the culling instead. Decide which outcome you want before you plant. Guppies also appreciate gentle filtration — strong current exhausts those ornamental tails — and warm, harder, alkaline-leaning water suits them better than soft, acidic water. Keep up weekly partial water changes, because a growing colony quietly raises the waste load month by month.
Signs the colony has outgrown the tank
Watch at feeding time: if the surface is a solid scrum and shy fish never get a bite, the count is too high. Nitrate that climbs quickly between changes, females dropping fry with no pause, and juveniles that never seem to grow all say the same thing. The fixes are rehoming juveniles to other keepers or to a shop that accepts them, splitting the sexes into separate tanks, or reducing fry cover so fewer are born surviving — every one of them easier than treating disease in an overstocked tank.
Keeping count as the colony grows
A guppy tank is a population, not a fixed display, so it rewards an owner who tracks it. As fry start appearing, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library — before the water quality makes the thinning decision for you.