Everyone starts off with a quota of 50 u2u's. When you reach 50, I'm not sure if you can not recieve new ones or if a new one would delete an old
one. But if you are reaching your limit, I would recommend you delete old u2u's.
If your looking to buy more, you can add 100 to you u2u quota in the
ATS
Store. This comes at the cheap price of 250 points, and I believe you can continue to add 100 more for another 250 points.