Thirteen

When Primus first created Cybertron, he created a crew of 13 living robotic beings to man him. These beings, in imitation of Unicron's transformation abilities, were able to change their forms into the likeness of vehicles, weapons, and machinery. They were the first Transformers.

Many of the 13 died in a cataclismic first battle with Unicron, where he was sucked down into a black hole and trapped along with his acolyte known as The Fallen, one of the 13 who had turned against his brothers and joined Unicron.

Of those that survived many abandoned their original functions and were lost to time. Only Vector Prime is known to still be following the path he was set upon by Primus.

The 13

The identities of the 13 remain shrouded in mystery.

Confirmed members are:

The Fallen (original name unknown)

Vector Prime

Prima (One story, Reaching the Omega Point, mentions Primon not Prima as the first Matrix holder, but this is never refered to elsewhere and subsiquent material has stated that Prima WAS definately the first TF.)

Possible hinted members are:

Maccadam (Rumor on Cybertron says that he is one of the 13 and that his bar exists outside regular space-time. The story Prime Spark from the TF: Legends anthology confirms this but is of questionable canonical status.)

The Leige Maximo (G2 establishes he was created at the same time as the first Prime, but implies that this was perhaps unintended, so leaving open whether he should count. Confirmed in Simon Furman's Alignment story as one of the first 13, but the story is "unofficial" and of debatable canonicity.)

Numerous other very ancient Transformers exist, including Alpha Trion (A3), Boltax, The Keeper, and the Last Autobot but none have been yet included in the ranks of the first 13, although a japanese timeline has A3 as the first Transformer successfully created by Vector Sigma.

It is stated in Beast Machines that the original transfomrers, meaning the original 13 had to learn how to transform, and that it took years of hard work and concentaion to do this.

From Teletraan I: The Transformers Wiki, a Wikia wiki.