start off with about a hand full,get them planted,
then over the course of time,add some more,
i think that you will be able to judge for yourself when you have enough,
take a step back from the aquarium and you will either think......
yeah that's cool looking,or........hmmmmm couple more will
be just right.you could always buy some plant ablets,
you place them in the gravel next to the plant and it will
take a look at other threads here,as there should be some
good advise on easy to care for plants,you can add some
different plants which will give extra cover.