There are 21 missions include tutorial.
If you want to complete all 21 missions, you need to play 4 times from start to end.
Here I list out flow of mission to make you easy when you want play it. When there is A,B,C,D, that mean it got different path of selection in mission. After you choose A, you not able to play B anymore until you replay the whole campaign from the first mission.

mission 1 is tutorial (give 1 yeoman archer)
mission 2A Defend the Oldwatch (give 2 citadelguard)
mission 2B Defend Grey Gate (give 2 yeoman archer)
mission 3 Join the riverwatch-light the fire (give 2 riverwatchrider)
mission 4A Defend farmlands-protect the herd (give 1 riverwatchrider and 1 yeoman archer)
mission 4B Arcane Lore-gather power (give 1 citadelguard and 1 riverwatchrider)
mission 5 Active the Golems-search the tents (give 2 runegolem)
mission 6 Clear Rookery-retake the rookery-roc rookery (give 1 rocwarrior)
mission 7A Defend Caravan (give 2 riverwatchrider)
mission 7B My pigs!-protect the pigs (give 2 runegolem)
(7A)->mission 8A Bandit king (give 2 bandit swordsman and 2 bandit crossbowman)
(7B)->mission 8B My Gods! - Defeat Ravos Ever Hungry (give Chaos Lord)
mission 9 Battle at the River (give 1 rocwarrior)
mission 10A Protect the people (give +5 Lore)
mission 10B Protect the Baron (give 1 kelleran cavalry)
(10A)->mission 11A Penetrate Defenses (give 2 riverwatchrider and 1 runegolem)
(10A)->mission 11B Retake Hernfar ISle (give 1 rocwarrior)
(10B)->mission 11C Defend the Tower (give 1 riverwatchrider and 2 runegolem)
(10B)->mission 11D Defend Zeiholt (give 2 riverwatchrider and 1 runegolem)
(11A/11B)->mission 12A The Blood Sister (give 1 Malaana)
(11C/11D)->mission 12B Defend the Bringer (give 1 Ravos)

so as you can see, if you want to complete all the mission 11, you have to replay 4 times- 2 times protect the people and 2 times protect the baron.
Hope that this can help those people as I had taken almost 1 day to find and list out all the detail here for you all.

Great job! I've been meaning to get around to this myself but was still missing 1 mission. Glad I don't have to worry about it now.