Canon 917 says that Catholics may receive Holy Communion twice on the same day only within a Eucharistic celebration (Mass) in which that person participates.
The Latin uses the word iterum which not only means “again” but more specifically, “one more time.”
If a Catholic goes to a morning Mass and receives Holy Communion, he or she can attend another Mass and receive a second time. Were they to attend a third or fourth Mass, they could not receive any more Communion that day. This prevents people from cultivating a numerical or quantitative theology, which the Church vehemently opposes.
One of the six precepts of the Church is that Catholics are expected to receive Holy Communion at least once during Easter season, if not more often (like once a week on Sundays, or even on weekdays).