Is this feasible? How many characters should they have?

1. 1 PC (with maybe a helper NPC.)

2. 2 PCs.

3. 4 or 5 PCs, basically a full party.

Depends on them!

I haven't been playing long enough to take on the role of multiple characters and stay invested in them like I should be. Or that might just be a result of how invested I get in the one character I am playing. Either way, that's me so asking me to play multiple characters wouldn't be good. I'm sure someone else could do it just fine.

I guess it depends on how immersed the gm wants the player to be, too. It he's telling a story about personal growth with a lot of emotional investment, then he probably won't be happy having all the main characters (the pc's) be part-timers. If he's telling a whirlwind story of incredible action with lots of wild combat, then maybe he'd be ok with a player playing multiple characters.

I'd rather play one character with an NPC but I am guessing one on one adventures are more work for the gm, too.

I think running a PC and an NPC is pretty simple in the this game. You could have an NPC wookiee, since they don't speak standard anyway, or an R2 unit. Either works as an additional player but doesn't have to really be played with as much attention to communication interpersonally.

Games with my son worked fine whether he had only one PC, or a main PC and a sidekick. The only real difficulty is giving yourself enough flexibility in combat scenarios...turning these into chases peppered with the occasional exchange of fire is usually better than if the PC tries to make a stand.

My main game is one GM (me) and my player. We use tons of NPC's, but, the only additional consideration/rule we use is that he rolls 2 Force Dices to recreate the Destiny Pool instead of 1.

I often run with just one player, with me controlling the rest of the crew. It's totally doable and fun.