Yes, this is allowable. There is no requirement to consider a certain number of applicants per posting. The scenario you described where the first 20 are reviewed and then adding another batch of a certain number if the first group does not yield an appropriate # of qualified candidates. The technique you use should be random sampling or absolute numbers in terms of deciding which ones to consider. If you use a different technique (such as closest zip codes, ranking of candidates, etc), it means there was a consideration of the application and you would not be able to say they were not considered.

You can use this OFCCP audit checklist to ensure you're doing what is required to maintain OFCCP's regulations including VEVRAA, Section 503, and EO 11246. Or request a demo to streamline your compliance and recruiting efforts.