The number of flowers in your town shouldn't affect your ability to grow more hybrids. I believe I read somewhere that you only get about 4-6 new flowers a day. To maximize your chances of getting hybrids, I would turn on the Beautiful Town Ordinance if you haven't done so already, so you don't have to water all your flowers. Then, just focus on a few patches of hybrids, water only them and use fertilizer.
With most types of hybrids, it helps if you already have two of them to use them for breeding, as they will usually give more consistent results than getting hybrids from two regular flowers.