Can I earn any more points after 21 recommendations besides adding new devices to my account, and the basic usage of the avast program?
Did you post that link to prove that the system is still in beta?
Also, but mainly to point you in the right direction (section/thread).
Well, since there are six pages, I rather not read through it all. Do you have any knowledge of what to do after 21 recommendations?
Not many people can give you an exact answer - as most people responding on these forums will be other avast users - and the scheme is still being evaluated.
For now, just continue as you have been - in this beta phase avast would be analysing how people are accruing points and over how long to be able to work out an appropriate reward scheme.
Your recommendations now equate/convert to points as and when the scheme is finalised you should be able to redeem them.