The round brush is the most versatile: its tip draws an outline, its belly fills an area. It is the one on school lists, for gouache as well as watercolour.
Single MSchool N°4, N°10, N°14 and N°16 brushes let you top up the pencil case size by size.
In sets, Lefranc Bourgeois Grippy bring three sizes together, 2, 8 and 12, 4, 10 and 16, or 6, 10 and 14. Pelikan Griffix come in 3 or 5, and the Pelikan Starter Set holds 10. The Colorino Kids Paint Set adds a palette to the brushes.
For a beginner, a three-size set is enough. Keep one brush for light colours and another for dark ones.