How often should we eat a day?

Well, depends on many factors. But for healthy people, up to 5 times.
The first meal, and the richest, should be the morning. And last meal should be at least 3 hours before bedtime, a simple and easy meal, preferably a tea, a fruit, maybe a yogurt or something easy.

Between these two meals, it should exist 3  different meals, at different time intervals. Whatever we eat these 3 meals should be nutritious and consistent with what we do on that day, according to physical and mental stress filed, with the state of our health, the climate, etc..

As a rule of thumb, a golden rule:
Vegetarian food is always healthier. If is bio, not artificially created.