This sets a number of individual matches that this rule should find. After this number of matches
are found, all other matches for this rule are ignored. For instance if a rule searches for matches
of "a" in "a a r d v a r k", and stops after 2, then only the first two letters here wil match; the
third "a" (the one between "v" and "r") will not count as a match.
The default StopAfter value is -1, which means it will not stop counting matches for this rule.
This function has some similarities with Max. However, with Max, if the maximum value is not attained, none of the matches will count.