It appears that Super Mario Maker will have a virtual cap to how many levels each user can upload online. This cap looks like it can be increased if the levels that are uploaded are ranked highly by the community.
This was a screenshot that surfaced today that displays the upload cap in the top right corner of the screen. There is speculation that you are awarded batches of 10 upload slots per medal you obtain and this user having four medals and 40 total uploads available would back that up. We do know that medals are earned through receiving high rankings from the community. This limit is most likely set in place so the Super Mario Maker servers aren’t filled with silly or garbage levels and is probably a smart move on Nintendo’s behalf when considering the future of the game.
Super Mario Maker will launch this September exclusively on the Wii U.