depends on how fast you wana harvest and how much space you have to work with. 5gal pots in a 3×3 you can run about 2 at a time depending on genetics. with 3 gal pots you can run about 5 or 6 plants in a 3×3 without over crouding. iv been running autos for 5months now and i like them.
i would NOT plant in solo cups. i have not tried it but i would not risk the stress. these things really are finiky with the roots. my gorilla skittles and sour diesels went into flower in 3 gal pots at just 12inches tall….. but have stretched 2x theyre size. so…… my 5gal runs added about 2-3weeks on harvest because they allowed for more space to grow. do not burry your seeds with direct water in the big pots if you go that route.