Limiting the number of records in a subform

[ORSP]

I have a form (frmContactInfo), primary key is ContactID. Within this main form is a subform (sfrmKeywordMatch), in which users can enter keywords (KeywordID) that match their research interests. I'd like to put a limit on the subform so that each user can only select 5 keywords from a set list.

I'm new to this, so can anyone give me some help?

Thanks in advance.