There's no built-in function today that exposes the number of NAs a stock encounters in a ranking system for use in buy/sell rules.
The closest workaround is to manually check each factor with IsNA() and sum them up:
eval(IsNA(PEExclXorTTM), 1, 0) + eval(IsNA(ROE%Q), 1, 0) + eval(IsNA(AvgRec), 1, 0) < 3
This is obviously tedious if your ranking system has many factors, and it won't stay in sync automatically if you change the ranking system later.
A native function along the lines of RankNaCount("MyRankingSystem") that returns the per-stock NA count for use in buy/sell rules would be a natural extension. Might be worth a Feature Request post if there's interest.