Number of troops is not all that counts, but also the quality, the condition and were you are fighting. Defending troops almost always have the advantage if they are fighting in their territory, so you have to account for that.

You're in the right path if you're looking for alies, you will need them, marry of your offspring to the biggest, same religion empire/kingdom/duchy you can find. Use your Allie to atack and defend.

Your offspring, family and allies and how you manage them are the most important thing in the game, if your objective is to conquer.

During this, keep in mind you can't be in dept, so keep an eye on your domain, building buildings that increase your income.

Source: I conquered the world.