1) update the software
2) install license
You do that in that way to avoid problems like the one that you are seeing right now.
So as soon as you upgrade to the latest software you should see the 50 APs available.... if you have installed the region domain license.
While everyone is correct, there was bug in the first release of the C35 firmware whereby the base license was incorrectly set to 25 APs ... vs the 50 APs that is should support. This was corrected in the summer, but there could be some units that are still affected. Please work with GTAC to get you to the right firmware level that will give you a base license of 50 APs.