Can I have multiple clients?

Yes, and it's even recommended! One of the best things about light entrepreneurship is the freedom to choose your own clients.

You can do a gig for company A in the morning and a project for company B in the evening. There's no limit to how many clients you can have. On the contrary, having more clients brings security: if one assignment ends, you still have other sources of income.

If you're also in paid employment, just remember to check the non-compete clause in your employment contract, so you don't work directly for your employer's competitor without permission.