Oh wait, do you already have shrimp and want to trade them in for plants? or do you have plants and want to trade them in for shrimp?
If you already have them, do you want to keep a maximum possible population before you start thinning? When they're all over the place- thin it out. The answer is probably somewhere between 150-300, but who knows for sure. Once I had 40 or so and the colony was breeding regularly, I stopped counting. Its going to have a lot to do with how you care for it. A tank with light feeding, fast growing plants and weekly 50% water changes of 50% will probably support tripple the number of shrimp than a tank with twice a month 20% changes.