Republic Commandos operate in four-man squads, but there have been some cases where several squads were deployed as a platoon, and there's evidence all over the place that sometimes one squad wasn't enough for the job. There's also this short story
that features a five-man squad; I'm not a fan, personally.
Delta and Omega squad operated with the Null ARC troopers, and towards the end of Triple Zero, MINOR SPOILER Omega picked up a new assignment with the Galactic Marines /MINOR SPOILER
. Regular clone pilots (though attached to the special forces brigades) transport them wherever they need to go. All squads eventually fall under companies (Delta and Omega are part of 05 Commando Battalion in General Zey's Special Ops Brigade).
All the training sergeants had batches of 104 commandos. How many RC-training sergeants there were we don't know (there were a hundred in total, but not all of them had their own batches). By a year into the war, however, over half of them had been killed. That, and the fact that they participate in a lot of classified/black ops missions, leads me to think that sightings of commandos are rare. If you see a commando, you'll either be dead in half a second, the mission's not classified, or he's on parade.