How many fish can I put in a 20 gallon tank?

Short answer

For freshwater: 6-10 small community fish (tetras, rasboras, dwarf corydoras) plus a single centerpiece (a betta, dwarf gourami, or pair of small rams). For saltwater: 2-3 small fish total - a clownfish pair plus one small goby or watchman is the realistic max in a 20-gallon reef. The old "1 inch per gallon" rule is wrong for both.

In depth

The "inch per gallon" rule was made up before anyone understood bioload. A 20-gallon tank can hold dozens of harlequin rasboras (1.5 inches each) but only one bichir (8 inches). Bioload is what matters - waste production, oxygen demand, and territorial space.

20-gallon freshwater stocking - what works

1 betta + 5-6 amano shrimp + 5-6 corydoras (CO2-injected planted tank)
10 ember tetras + 6 dwarf corydoras + 1 honey gourami
1 pair apistogramma cacatuoides + 8 cardinal tetras + 6 hatchet fish
1 Bolivian ram pair + 8 black neon tetras

20-gallon saltwater stocking - what works

A 20-gallon nano reef tops out at 2-3 small fish. Realistic options:

1 pair of captive-bred Ocellaris clownfish + 1 small goby (Yellow Watchman, Tailspot Blenny)
1 captive-bred Banggai Cardinal + 1 Firefish + 1 Yasha Goby
1 small wrasse (Six-Line or Possum) + 1 cardinal

The bioload reality

Saltwater fish produce more waste per inch than freshwater fish, the water holds less oxygen at higher salinity, and the smaller water volume of a nano amplifies parameter swings. A 20-gallon reef with three fish runs cleanly; a 20-gallon reef with five fish struggles.