Sorry didn't try to be rude
And there is actually infinite number of missions. It takes multiple missions sometimes, unless you wrack up a ton of kills in the mission, to level your characters. But you have four guys each with their own exp, and then two others who you don't use you wont be generating exp. Only character you have to use is Force Commander, who has all the attributes evenly.
You either spend attributes in health, melee, ranged, or energy. Health, besides raising the health obviously, gives you some neat upgrades. Takus gets his squad members upgraded to veterans, and get that sweet white armor. Energy raises their energy which what they use to spend on abilities, and also neat upgrades. Davian Thule gets Vernerable, so you can be a Vernerable Dreadnaught, making you much stronger.
Melee raises melee damage and gives you melee oriented abilities, as with range. One of the best abilities is Avitus, the devestator marine, who can make him not need to "set up" his heavy weapon. He just runs and stops and shoots that heavy bolter, or plasma cannon, instead of having a wait time for him to start shooting.
Tarkus, and Force Commander all have Terminator Honors. Avitus, and Thaddeus can learn Terminator Honors in the melee skill tree for Theaddeus and health for Avitus. Later on in the game you get terminator armor and it transforms your unit into a terminator squad. Thaddeus, being the Assault Marine, because Assault Terminators.
I love the single player, and its fun to play it again even to try out different things, like giving your force commander a heavy bolter and tac marines power swords