Hi
@TheTash04, welcome to the forum.
You can add any number of sims to a save game, and it's not that hard to do. I'm going to assume that the sims you want to add are already in your library waiting to go.
1) Load the save game you want to add them to.
2) The map screen where you choose which household you want to play, navigate to the town you want to put them in (Newcrest in my case), then open your library with the button in the top right corner
3) Pick your family from the library and click the 'Place Household' button in the bottom right
4) Choose the lot you want them on and 'Confirm' the move, again in the bottom right
5) Repeat with any other sims you want to add.
6) Go back to the family you want to play and have fun seeing your other sims walking around.
The sims you have added will show up on the map screen as having been played, it this bothers you they can be set to unplayed through Manage Worlds.
1) Choose the town and lot you added your new sims to
2)Click the More button in the bottom right (...), then Manage Household to pull up the details of that family
3) Click the plumbob on the left to set them as unplayed
Important final step: Enjoy your game
