the way we did it was one/gallon
my buddy got 5 and i got 7 for my 20 gal
it worked fine for us
we both ended up getting a pregnant one too but i dont think that worked out too well
as for drift wood i unno but mine seemed to like the "Cave" like thing i had in my 20gal and they hid in the castle i used to have in it a lot too
shrimp are pretty nifty critters and are easy to care for. i dunno if betta like them but when my buddy would give his fish a bloodworm thing the shimp would go nuts for them. as would the fish. with jsut one betta though you'd prolly have to cut it up or melt it first and serve it in extremly small portions.
just my two scales :)
PS: Just to clear something up! i didnt mean ALL FEMALE BETTA ARE UGLY. there are blenty of amazingly colourful and beautiful ones. as an example the one in this thread is amazing. white with touches of colour on her fins. BEAUTIFUL! what i ment was the betta walmart, thepetstore, AJ's Fins, and auqazoo here in my city are ugly. they're colour is faded, they are lopsided, burnt, water is yellow/blue/brown. they just seem to be sick and dying. they dont take care of them. i didnt mean to be offensive i just ment to say that the peeps at my petstores dunno what they're doing and are killin their betta and making them ugly :)