thanks. As it is a teenage girl and her first tank I will tell her to start with 2 see how she goes. If she does ok then put more in. I don't know if she will be going down the plant route. Personally I would but not my tank.If only guppies, I would go for maybe 8-10.. if your water change schedule and water conditions say it's fine (as always, follow test results) if you have thriving plants, then possibly more.
But follow test results to know, and don't slack on water changes if you stock on the heavy side.