To make a game on Amiga you need a music/sound guy, a graphics guy, and a programmer. No more, and sometimes less
It's certainly no impossibility for 1 coder to make a nice game with test-gfx and test-music/sounds. How many people need to be involved usually only depends on how much graphics, how many levels, etc.
Say 6 months-1 year for a game with at least a little ambition. But 2 coders locked in a room for 3 months - now we're talking magic and good times! hehe