I have a quick question about how the stack works in battle.

I just played my first game and had the Ultramarines Captain Adeptus Astartes.

His special ability is a battle bonus of +2 .

I also had the mission card Report to the Blackstone Fortress which has a reward of being able to put a character token on the card and it gains a battle bonus of +2 .

If my character also has a Storm Bolter (wargear) with +3 and my strength is 7 , would my battle score be:

7 + 2 + 2 + 3 + Die roll ?

Do ALL 3 stack, or do I have to choose eith the Storm Bolter or the Character token, and not both?

They all stack, of course, you'd have to spend a charge token off the Storm Bolter to get the +3. Good thing about combat in Relic is that if you end up rolling low and your opponent hits a 6-string of rolls, you can still get beaten (or beat someone with higher stat despite the odds being grim). In Talisman, you'd not even bother rolling most of the time.

I have a quick question about how the stack works in battle.

I just played my first game and had the Ultramarines Captain Adeptus Astartes.

His special ability is a battle bonus of +2 .

I also had the mission card Report to the Blackstone Fortress which has a reward of being able to put a character token on the card and it gains a battle bonus of +2 .

If my character also has a Storm Bolter (wargear) with +3 and my strength is 7 , would my battle score be:

7 + 2 + 2 + 3 + Die roll ?

Do ALL 3 stack, or do I have to choose eith the Storm Bolter or the Character token, and not both?

The only limit to stacking is that you can only use one weapon and one armor. There are no limits to the other types of assets.

I have a related question:

there is a power card that doubles your attribute value. In this case is the max. attribute still 12? Example: Strength = 9. Is strength in the fight 12 or 18?

It would be 18 as you can use multiple types of adds to get your attribute above 12. The rule for your starting attribute is that you can not go over 12 even if you get a something that permanently adds to your stats. So if you had the power card and the max stat it would 24 and then start adding after that.