How you do it depends on the situation. For a newly set up tank I like to fill the tank with as many plants as possible. This along with adding mulm from an established tank instantly cycles the tank. In an established tank, the more sensible way to add plants is a few at a time and then propagate from there with the ones that do well for you.
Stem plants are easy to propagate. Just cut off the tops and replant. Rosette plants multiply in other ways. Some plants send runners and others multiply or enlarge through rhizome growth etc. and here more patience is necessary.
Some plants just don't grow for me while others flourish in my tanks. So when buying different plants, I just get a few and see how they do for me. Rather than fight the system, I chuck the ones that don't do well and move on with the ones that perform.
Len