I think two is a good number, *maybe* three if you have the time and money. I know I couldn't take care of more than three, I'm way to busy to give attention and care to any more. Technically I only take care of 2 1/2 because one of them is my mom's haha, i just do her water changes for her =)
My Current bettas:
Sullivan- Orange rose tail halfmoon male
Daisy- Mustard Gas (black and yellow) veil tail female
Milton- Teal butterfly halfmoon male (mom's betta)
DFTBA to nerdfighters working to decrease world suck!