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.
