How many missions are there in the Resistance campaign?

The OFP: Resistance campaign contains 20 missions:

1. Invasion
2. Crossroad
3. Contact
       or
4. No Turning Back
5. Ammo Low
6. Scrap
7. Field Exercise
8. First Strike
9. Hostages
10. Information
11. Occupation (A)
        or
12. Occupation (B)
13. Counterattack
14. Contraband
15. Reckoning
16. Fireworks
17. The Pass
18. Hunting
19. Trap
20. Fire Fight

Whether you get to play the Contact or the No Turning Back mission is dependent on the outcome of the previous mission, Crossroad.

Whether you get to play the Occupation (A) or the Occupation (B) mission is dependent on the outcome of the previous mission, Information.

(With thanks to Mr. Nameless for the corrections).