At most I'd say at most 318 active IIIs as we know, I think 12 of Gamma are dead thanks to Halo: Ghosts of Onyx, the rest are unknown. EDIT: Make that 320 IIIs because of the survivors of Beta

The ORION Project was ONI's original supersoldier program, which you already know. Essentially it was a bunch of slight genetic and drug-based alterations to regular soldiers, making them above human, but not to the extent of the IIs, IIIs, and IVs. IIRC, they went on two missions and were never seen by the enemy in either of them, with only one of them lost. It was shut down because the results were not as impressive as ONI had hoped. Decades later, Halsey based the SPARTAN-II Project on the information gained from ORION, renamed SPARTAN-I.

S1's were given slight augmentations, mostly drug-based. Didn't work out too well. Johnson was an S1. Not sure how many S3s are still alive. Ones from the Dyson Sphere, Grey Team(?) and Team Katana that I know of.