Historical Rules · 17 April 2020
Rule 122.7
Official source text from the 17 April 2020 Rules release
An ability that triggers “When/Whenever the Nth [kind] counter” is put on an object triggers when one or more counters of the appropriate kind are put on the object such that the object had fewer than N counters on it before the counters were put on it and N or more counters on it after.