One has to remember that Sauron made the One, the seven and the nine. He made seven for the seven dwarven kings and nine for each of the great kings of men at that time. He made the one ring to rule the others so he could have dominance over those races. He didn't make the three eleven rings - the rings of fire, water and air were made by the elves. Indeed, the Eldar made many rings, but only the greatest three were saved from an attack by Sauron and they were thereafter hidden. So, Sauron made the One mainly to try to obtain mastery over the elves through their rings. It didn't work, because the elves perceived his plan and took off their rings and hid them.
From this we can see that Tolkein didn't so much miss out "5". Sauron made one, seven and nine for historical reasons, and the elves made many of which 3 survived. The apparent odd number sequence with a gap is therefore not really a reality. The seven dwarven rings don't survive long anyway - Sauron reclaimed 3 and has them well before LOTR and I think 4 were destroyed by dragons.