sigh....i keep 5 currently in 2g bowls BUT i do a rotating daily 100% water change (bowl gets cleaned and refilled every 5 days) and replenish water from 38g or 75g cycled tanks...there more work than the larger tanks...i had 7 but wouldn't go more than that...my bettas average 4 years life spans from time of purchase and that's including the odd suicides...
my point is the only way i feel they can be kept in bowls is tons of water changes...i like to think using cycled tank water helps a lot but that's just a theory of mine ..aged water would probably work too