As far as I know the number can just keep increasing, but superfresh is the highest actual rating. Once you've hit superfresh on a weapon you won't gain any further freshness rewards on it (even dropping down and rising back up again) but you can still earn those big xp bonusses by pumping up your freshness on other weapons, seeing as each has it's own unique rating :3
Oh I see! Guess it's time to start learning some different weapons than.
That's actually a nice incentive.
Thanks!