I don't think it matters how many plants you are using for a project, residential, commercial shouldn't matter. Most nurseries won't give a discount if you buy one or one thousand plants. Everybody has a different way to mark up their plants. Like said above. Price so you make money and not so much that you won't get the job. There are so many variables that go into pricing landscape jobs that no one on here can accurately tell you what you should charge. But there are some good threads that will give you an idea of how you should charge. Good luck.