Those monks who follow the pātimokkha (Sk. Prātimokṣa), either Mahayana or Theravada, keep this rule (or, in certain cases, a rule to eat only one meal per day). The rule is simply:
37. Should any bhikkhu chew or consume staple or non-staple food at the wrong time, it is to be confessed.
The wrong time is generally understood to mean outside of the morning hours between dawn and noon. So, technically, one could eat ten meals during this time if one so desired. The Buddha himself enjoined a single meal, as stated in the Bhaddāli Sutta (MN 65):
“Bhikkhus, I eat at a single session. By so doing, I am free from illness and affliction, and I enjoy lightness, strength, and a comfortable abiding. Come, bhikkhus, eat at a single session. By so doing, you too will be free from illness and affliction, and you will enjoy lightness, strength, and a comfortable abiding.”
Later, the monks were concerned about eating rice soup in the morning, thinking (I assume) that it would be considered a second meal. The Buddha enjoined them to partake of it, citing physical benefits (can't find the source right now, will try to add it later). So, it is generally understood that one should eat a single meal before noon, but may partake of something light at first light of dawn as well.
Many monks these days stretch or even break these rules, drinking milk, soya milk, etc. or even eating in the evening, but those who follow the rules, in both Mahayana and Theravada, will generally stick to one or two meals per day.
Eating once per day is actually specified as an optional (dhutanga) practice, meaning one would forgo even rice soup in the morning.