I would not doubt there is a cap for counter, and really it is'nt hard to get 75% counter with just merits/traits/counterstance/melee gaiters. Go into Abyssea and you can get another 20-50% (assumed estimates) counter from Atma, and Saraphi/Cross-counters and anything else is really just icing on the cake.

As Drhatchet said, accuracy also plays a big part in the success of countering. Since accuracy has a hidden cap (90% for 1h and 95% for 2h I believe) it's realistic to think you cannot have 100% counter no matter how much you stack on.