If the question was "how many CAN I keep", the answer is really "as many as fits in your tank". Clowns are very social and the larger is the shoal, the better it is for them.
The same applies to other Botias to various degrees. For example, I keep a shoal of 13 Kubs right now (2 more live with the clowns until I have the energy to catch them), and they appear much happier together than when they were in separate tanks.
3 foot tank will not be enough at some point...when the fish reaches 3"-4" it is better to give them a larger tank. 6' is really the smallest size for subadults.
And as for the minimum number: my feeling it is really 5 for the clowns. In all cases, a shoal of fewer than 3 should not be kept other than in a quarantine tank for a limited time.
hth
PS. Per growth rate: for clowns specifically, the rapid growth period described by LL means about 1"/year. Could be faster in good conditions. So if you have 1" clowns now, figure you must have a larger tank in 3-4 years.