Under normal circumstances, twice: “A person who has already received the Most Holy Eucharist can receive it a second time on the same day only within the Eucharistic celebration in which the person participates, without prejudice to the prescript of canon 921, §2” (Code of Canon Law, 917). “Participates” means normal attendance at Mass as opposed to just showing up for communion. (Note: Canon 921, §2 concerns those in danger of death.)