I Want to replace an existing unused 20 amp two phase electrical outlet with a sub-panel containing single phase 110 volt circuits with appropriate circuit breakers. My question is: How many separate circuits does the NEC allow in this type of sub-panel?

When you say "two phase" , do you mean a single phase circuit with two hot legs creating a 240 volt 1 phase circuit? Just because there are two hots, does not mean it is two phase.

If so, and the circuit/outlet is rated for 20 amps, any subpanel you install can have up to and no more than 42 circuits, however, the connected load can not exceed 20 amps, due to the rating of the feeder.

Seems your limited to 20 amps.