Whats the # 5 printed on the tokens mean?

Thanks

Age

It means that it adds 5 strength to its owner when you are calculating who wins the battle. The reverse (damaged) side says 3, meaning a damaged stronghold only adds 3 strength.

Strongholds are great ways to defend areas with somewhat less forces. A Conquest order can help balance the strength out for the attacker, though, but if it's at full strength, an undamaged Stronghold still provides +2 net advantage to the defender.