How should i create swarms, I would rather have my players fight a swarm of Sahuagin then 12 of them, should i keep doing what i already do, multiply the health by 2?

there are several swarm statblocks you can look at for assistance. rat versus swarm of rats for example.

Sahuagin already have a swarm creature, the Sahuagin Hatchlings - a swarm of tiny beasts from Ghosts of Saltmarsh 

or do you want them to fight multiple medium sized creatures? 

I would take a look at the skeletal swarm to see how swarms of Merium creatures should work.

Medium creatures, and i don't own ghosts of saltmarsh

okay, are you asking for help on rules for Mob combat, or do you want to create a group of medium creatures with which you will apply the standard combat rules?

Handling Mobs is found in the DMs guide under Combat in Chapter 8, but it’s not great

grouping medium monsters as a “swarm” could be done but I think what you are doing is okay by making them fight a couple of harder creatures. Either select higher CR monsters or add to their AC and/or HP and describe them as being big muscly & hard scaled  

standard combat but i've figured out a solution Thanks for help

I'm not sure why but a lot of D&D content people on blogs and YouTube have been talking about 4e's minion rules quite a bit as of late.  If you type up D&D minions 5e into your search medium of choice you'll probably find some ideas that might get to what you're trying to do.