It depends on the size of the plants, of course! If you buy from an online supplier like Java Plants, for a 15 gallon tank they supply 50 plants, which sounds outrageous until you realise that most of the plants are the size of a beansprout and at least 50% of them won't see adulthood.
I'd stuff as many plants in your tank as you like (provided you have plenty of light) and when it's full, stop.