You need at least 6 good players. 3 to attack and 3 to defend when it's the other team's turn to attack. If you're getting sandwiched by 2 factions bump that number to 8. You could attack a 2v2 and have 6 left to fend off two attacks.
The main reason Empire lost the testing phase of Planetwars imho, is because we were reduced to relying on 2nd rate players (like myself) to supplement the A-listers. Factions like the Federation have so many top players that they basically could clock in whenever they wanted and there was always somebody. Whereas on our side it was basically the PROs and Rymarq and Aquanim that were around. I think our top players were frazzled by having to basically fight around the clock.