The figure 13 was quoted by the FBI as coming from "street sources".
One murder he was most definitely involved in was that of Thomas "tommy" Ernst, boyfriend of terri D'Alessio, daughter of John "johnny D" D'Alessio, brother of Michael D'Alessio, Billotis captain.

Ernst, also a D'Alessio crew member had done something to piss off his girlfriend dad and uncle and was slated for death.
The first hit attempt was a mess, they tried to run his car off a rural Staten Island road and took shots at it, but the hitters fucked up because Terri D'Alessio was in the car with tommy and she ended up returning fire at the would be killers.
There was one or two more attempts on him before he was finally mowed down in his driveway. The police at the time identified Billoti as the shooter.

He spent a lot of time as Alex DeBrizzi's cauffer/bodyguard. DeBrizzi being the brother in law of Michael D'Alessio who was married to Marie DeBizzi. There is an old FBI report of Bilotti being shooter in two hits for DeBrizzi although I could never find the victims names.