One of the most important aspects of Kung Fu is ‘Bian Hua’ That is Changeability. THe ability to change from one move into another.
If you only practice a few forms you find you will be very stuck with those movements. To Break this you can practice the individual ShouBa of the forms (that is the individual movements) and practice free combining them. ALso you can try more forms.
If you train a great deal, then there are only advantages to training more forms. It makes you very comfortable at changing and gives you such a large repertoire of techniques that you see through the move to the formula behind and no longer need any of the moves at all. Thus you can become formless. You can also become this way by having the style explained to you in very high detail, but such a teacher is rare.
When you learn a new form it unlocks new information about all the older forms you learned. So you will find you progress more quickly generally. Its all about time. If you have a lot of time to dedicate, then I would suggest learning as many as you can get your hands on. However few people have such time, and so it is important to sreamline. In this case I would suggest learning enough for a complete style but to focus on their individual techniques.
In SHaolin the problem for a lot of beginners is having a complete martial art. The problem is that WuBuQuan, LianHuanQuan and XIaoHongQuan are not enough. THey don’t contain all the techniques you need. Neither WuBuQuan or LianHuanQuan are old Shaolin forms, they are later additions specifically for beginners. XiaoHongQuan is a complete stlyle if you learn all its roads (4 forms). It can work in combination with Pao Quan very well as well. However it is not so well combined with TongBei, Luohan, Jingang, etc.
No one has yet organised Shaolin into a Logical syllabus. So at the moment, in order to learn the style well, you really do have to practice a lot of forms. Or have an extremely good teacher who teaches you all the principles well.
IMPORTANCE
So, is it important to have the knowledge of many forms? No, it is important to have the vast knowledge of your system of Wushu. This may be just a few moves or even a few principles, well applied. However in Shaolins case, its gonna take quite a few forms.
It goes without saying that a form should be well learned before moving on, but not to the point of stagnation. It is difficult to know when to move on. But you will know yourself. If you get to a point where progress is slow, move on.