What level of projects can I expect to achieve after finishing the CSS basics course?

I think that one of the great things about coding and one of the beasts is that you just have to accept that you're never going to know it all. You just won't. That has more to do with everything being in a constant state of change than anything else.

My advice to you would be to start on a portfolio site. Ideally, you're going to need one eventually, right? Break it down into the smallest imaginable steps that you can think of. Make a list that contains things like "decide color scheme", "make wireframe", "add navigation bar" and start checking them off one at a time. You don't have to host it anywhere if you don't want to. Keep it private to yourself for a while.

It helps to reinforce what you've learned. I'd be willing to bet that during this, you will at least have the thought float through your mind a few times "I wonder how I would make this ... look like this?". It's the curiosity questions that will drive you forwards. You might find a copy/pastable answer on stackoverflow. You might not. And even if you do find one, you might not truly understand it.

Keep going forwards, but keep some little side project on the side that you tinker with ideally once a day, but at least a few times a week. I'll bet at the end of a month or two, you'll be amazed at what that has formed into.

But back to your original question: what can you do? Whatever you like. You've learned the basics, but it's up to you to take it further. Use what you know now and expand on it as you go.

Hope this helps!

It honestly depends on the kind of projects and their complexity. So long as you know where to look for help and have a basic understanding of the language you don't really need to take extra courses unless you have issues understanding certain CSS concepts or want a refresher. For example, if you're unsure how to do animations without using @keyframes there might be a course on here to help, or other markup sites you can use to learn.