How many members of the family of the element fluorine have?
There are six members of the halogen family: fluorine, chlorine, bromine, iodine, astatine, and ununseptium which has not yet been named.
The single "most likely" element that would form an ionic compound with fluorine is cesium, or possibly francium if enough of it could be collected. This is because cesium, among stable elements, has the lowest electronegativity and fluorine has the highest electronegativity. However, any alkali or alkaline earth metal element in fact readily forms an ionic compound with fluorine, as do many other metals.