CincMace is correct in insisting you use the applicable code in your country.
IBC2006 no mention on the number of doors
NFPA92A - no mention on the number of doors
For UK BS5588 - ONE (Class A & C System)
TWO (Class D System)
THREE ( Class E system)...
Some good references for stairwell pressurization are:
1. Design of smoke Management Systems by ASHRAE
2. Smoke Control by Pressurisation by Wild (from Woods Air Movement Ltd.)
The one from ASHRAE is used for countries using U.S. Codes while the one from Woods gives comparison of codes from...