One.
A mated pair of regani could work, but it takes a much larger tank to get them to pair successfully. They don't do well with forced pairings either. Ie throwing any male and female together like you do with convicts. The best idea would be to get a group of regani and throw them in a larger tank and let them pair naturally.
Many people have bred regani and compressiceps in 20 longs (same length and width as a 29) but most have paired them in the larger tanks first.