In The Sims Mobile you can only play with a single sim at the beginning. Only in time will you unlock more space in your home. The second space will be unlocked for free. You just have to click the icon of another sim after reaching level 3. You will be able to create another one.
However, unlocking the 3rd and 4th slot also requires spending sim cash. Third slot costs 175 Sim cash, and fourth - 515 Sim cash.
In The Sims Mobile you can switch between sims at any time, even if they're in different locations. There will be a short loading screen in such case. To switch the sim, you just need to click its avatar.
If you're playing with more than one sim, you should expect to get more money and advancing fasters. Playing two or three sims, you can switch between them no problem. Thanks to that, you can always aid your sim. They can all work on different hobbies. You can switch between them clicking the finished action icons. This makes things a lot faster.
Careful who you're controlling when confirming completed events - if you do that with a different sim that the one, who actually finished the action, the points will be directed to the first one.