Flow requirements are usually based on two consecutive floors and the main egress doors. So, 3 doors.
You'll need your country's fire code - there are lots of influencing design factors in this design - maximum pressure in the stairwell, max door opening force, presence of smoke stop lobbies, flow velocity through doorways etc.
We have been wrestling with the SAME dilema for the last 6 months with no clear answer. We, as designers, want to do what will work better for occupant evac, but are pressured to keep it cheaper.
I got some answer - 10% of the number of doors or a minimum of 2 doors to be considered as open. But, couldn't didn't get any cross reference with any code.
We have 4 staircases and 10 floors - that makes it 40 doors. We have considered 1 door in open condition in each staircase. But, no code details to support the above or otherwise.
When sizing the fan don't forget to add in extra leakage of doors and shaftwall. I did one where the previous engineer figured a 5 hp blower. That was very inadequate. then he tried a 10 hp. That didn't work, it was too small. We took over the project and put in a twenty hp, which seemed to work well. Don't forget to wire it to emergency power, hope your generator is big enough.
I've always tried to convince clients to go for vsd's. Makes life much easier to get the system balanced. Door closers need to be checked first in existing buildings - nothing worse than spending two days in a stairwell trying to get the bugger working to find half your closers are shot.
50 Pa is a great number for controlling pressure (and the standard in two countries I know) as it also coincides with the standard leakage airflow test pressure, meaning data is available for different construction types and much is written by people like Max Sherman at the LBNL.
There are 4 components which add up to the total air quantity requirement for pressurisation :
1. Stair volume x 10 Air changes per hour.
2. 3mm gap considered over the perimeter of each door and with a 50 Pa pressure difference, air quantity that would leak out is calculated. Formula based on ASHRAE.
3. Air that would leak out through wall cracks based on an average leaky building - again wtih a 50 Pa pressure difference. Formula based on ASHRAE.
4. Assuming 1 door completely open and with a velocity of 0.8 m/s.
I have added all the above and arrived at the total air quantity required for pressurisation of the stairwell.
The big assumption I am making is item no. 4 - i.e. one door open, for which I don't have any back-up in terms of a code, as my local code doesn't give enough information.
You need to work to a code. Your assumptions are wildly different from any code i've worked to. One door and 0.8m/s seems too light. As an example, take a look at the Singapore Fire Code section 7.2.
Your own local code obviously takes precedence over this. As a life safety system, our design will need to be signed off by a PE so you may as well get the information up front.
Find out your normal infiltration ACH for the space, multiply by 20 for 50Pa ACH (refers LBNL model).
You probably don't need to run normal stairwell ventilation during fire-mode, and 10ach is a lot anyway.
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)
THREE ( Class B System)
For Canada NBCC - FOUR
For Australia AS1668 - THREE
For Singapore CP13 - THREE
However, certain door velocity is required for each recommended number of doors.
