Is there a limit to how many designs I can upload?

No, keep uploading – you can add as many designs as you like to the studio so please utilise any old designs you've not sold or keep busy designing. However it is advisable to not upload a large quantity of designs at once, if you plan on uploading lots of designs stagger it a bit so do 10 every few days - this will increase your time at the top of the 'New Arrivals' page.