Does the number of valence electrons affect chemical properties?
Speaking bluntly, Chemistry is all about valence electrons and their transfer in reactions.
The Periodic Table contains different groups ( the vertical columns going from left to right) and in all of these groups the one thing the elements have in common is their amount valence electrons. (amount of outer electrons)
As you may or may not know, elements from group 1 and group 7 (aka group 17) are both very reactive, but in different manners. Group 1 elements are reactive because they easily "lose" their 1 valence electron to gain a stable octet ( a full outer shell, that is). The group 7/17 elements have 7 valence electrons and gains 1 electron to gain a full octet, thereby being reactive in that manner.
So there is a direct link between valence electron number and reactivity of an element, Where Flourine (F) from group 7/17 and Cesium (Cs) from group 1 are the most reactive elements in the periodic table, because of their ability to either "strip" other elements of their electrons (F) or to force their lone electron onto something else (Cs).