Might depend on the context... my first guess would be "Are you dating other people?"

You've got a couple of options depending on context. Seeing a lot of people could mean that you've looked out the front window of your workplace and there's a queue of people waiting. You literally see a lot of people. The other option is that seeing can mean dating. So if you see a guy, and you're talking about him to a friend, then you might say that he's seeing a lot of people. This would be because he has many girlfriends etc.