Nope! Upload as much as you want, when you want, and create an awesome store!
The only catch is that you are only able to upload 50 designs per 24 hours. This means, if you upload 30 designs at 1:00 PM and 20 designs at 3:00 PM, you will be able to upload another 30 designs the next day at 1:00PM, and 20 designs at 3:00 PM, or wait until 3:00 PM and upload 50. Each click on the "Publish" button counts as one upload, so please be patient and try not to double click this button even if the uploader is being a bit slow.