Tin has 4 valence electrons.
The quick answer here is that because tin, Sn, is a main-group element, the number of valance electrons will be given by its group number.
Tin is located in group 14 of the periodic table, which means that it has 4 electrons in its outermost shell, i.e. 4 valence electrons.
Now, you can prove that this is the case by constructing tin’s electron configuration.
Tin has an atomic number of 50, so right from the start you know that its electron configuration must account for a total of 50 electrons. You will have
The fifth energy level, n=5, is tin’s outermost energy shell. As you can see, tin has a total of 4 electrons on this level
2 electrons in the 5s subshell
2 electrons in the 5p subshell
Once again, you can conclude that tin has 4 valence electrons