For the most part, you can just look at the number of the Group in which you find in the element. If you’re using a modern periodic table, the columns are numbered Groups 1 to 18. For the first 10 groups, the number of valence electrons on an atom equals the group number, and for Groups 11-18, it’s Group number minus 10.

Valence electrons are those that get involved in chemistry. They might be lost or gained or shared to form bonds, or they might just sit in the frontier orbitals of a compound and change the properties, but the # of valence electrons are those that determine the chemistry — which is why the periodic table, acting as a map to valence electron configuration, is such a useful organizing tool. A valence electron count is always the starting point for any kind of explanation of chemical behaviour. As you move across the table, you add one electron to the valence shell for each element, until the shell fills at Group 18 and then you start over with a new row.

Thus, Li, Be, B, C are in Groups 1, 2, 13, 14, and they have 1, 2, 3, 4 valence electrons respectively.

Mo is in Group 6 (under chromium), so it has six valence electrons. There are 42 electrons total, 36 of which fill up the “core” orbital shells — too low in energy and too contracted in size to directly take part in chemistry or bonding, which leaves six to play with. The ground state electronic configuration for a Mo atom is [Kr]4d(5)5s(1) — so there are your six valence electrons.