If I add a golden level 12 hero to my ACP it adds around 2,000 troops to my front line. But if I add..

Oh nevermind I found the answer to my own question for those who had the same problem go onto your hero's skills and select the first one, one hero (in my case) was upgraded on that and the other has not. Hope this helped 😃

That's right the amount of troops added depends on the level of the first skill and also the 6th skill. Orange heroes will allow you to have more troops eventually once these skills are unlocked and upgraded.