I will help my customer (a local printing huse) to create a pdf-template with formfields for quotations. On your support page I can read that there are a limit set to collect 500 filled in forms.

There is not a limit if you Reader-enable the document with Acrobat DC, as opposed to previous versions. But you should not Reader-enable documents for others as it would violate the "no service bureau" part of the Acrobat license agreement. Note that the desktop versions of Reader 11 and DC can save non-enabled forms, as can mobile versions that are forms-capable, so Reader-enabling is less important as users adopt these more recent versions.

Why is there this limit, is a good question. The answer is that there is a premium enterprise product, which has a higher limit, and they want to sell this.

DISCLAIMER: I'm not a lawyer.

This is a legal limit, not a technical one. If they collect more than 500 responses they'll be breaking the terms of the EULA for Acrobat.

However, this limit only applies if you apply the Extended Reader Rights to the file, which is not necessary if the file is being filled-in using Adobe Reader XI or higher (or Acrobat, of course).