Im just wondering, if someone had a lot of demons in them and sins against God. Is it always counted as mortal?
What do you mean "counted as"? We are not excused from decision-making because of demonic possession, if that were to be true. However, demonic possession is much more likely to occur in those who are in a state of mortal sin. We welcome the devil in when and after we commit a mortal sin because at that moment we have driven God out and weakened our defenses against the devil's reign within us.
Demonic possession is rare. Mortal sin is frequent and unfortunately common, but a sincere Catholic should make his or her first priority efforts against his own mortal sin, for obvious reasons but including the protection from demonic possession. First line of defense: sustained sanctifying grace.