Is there a limit to offering shifts?

Hey y'all! So I just had a couple of questions about offering shifts on the market place. 1. Is there a limit to how many shifts that you can offer? 2. How can you tell if someone picked your shift up? 3. I try to offer a shift next week but for some reason the marketplace app does not let me, how can I fix this? (Sorry if these are basic questions this is my first time offering shifts)