I never thought much about how many vegetables to serve at a meal until I read recommendations in a hundred-year-old home economics textbook, but my first thought was “the more the better.” That’s not exactly what the old book said:
Too many vegetables should not be served at dinner; the general rule of serving two is a good one to follow. Lettuce is usually served with any salad and would make the third. In choosing the two, it is better to select one starch and one green vegetable; the two being pleasing in taste when eaten together.
Elementary Home Economics (1921) by Mary Lockwood Matthews