How many times in one day can a layperson receive communion?
Twice. The Code of Canon Law of the Church states:
A person who has received the Most Holy Eucharist may receive it again on the same day only during the celebration of the Eucharist in which the person participates, with due regard for the prescription of can. 921, part 2.
Thus, a person may receive twice in one day. However, the second time must be at Mass. The only exception to this rule is the one mentioned at the end of the canon which refers to receiving communion anytime when in danger of death (called viaticum). Another exception I can think of would be for a minister bringing communion to the hospital or to a shut-in. Often the minister would have attended Mass and received communion earlier, but be required to again receive communion by consuming the Eucharist that is left after their visit. Yet another exception to the rule is the priest who by necessity must celebrate more than two Masses on a Sunday.
The reason for such a rule is simple. There is infinite grace in the Eucharist, and although we are finite and our receptivity to grace depends on our own disposition, we should not be seeking to quantify or multiply graces by receiving communion more than once a day. The Eucharist is not a “thing” that we can get more or less of. It is a Person, Jesus Christ, and when we receive Him once in a single day, that is already more than we can comprehend. Because of situations such as a funeral or wedding Mass in addition to daily or Sunday Masses, there are times when we attend Mass more than once in a day and would like to receive communion again. This is permitted, but the Church respectfully restricts reception of communion to a limit of twice in one calendar day.