Small bands?
The entire army strength in Spanish Morocco, from the start. Took control of the territory almost immediately, leaving the rebel commanders there free to send as many of their men as they could transport to Spain to fight. That gave the rebels control of the largest army command, by far, from Day 1, with the best troops, & no local opposition, so it could be deployed wherever it was needed.
The proportion of the peninsula army they controlled after the first day or two is disputed, but with the troops from Africa they controlled well over half the total army, & most of the rest had been disorganised by the rebellion.
There were eight military districts in mainland Spain. In five, rebels more or less took control of the military command, or already ran it (e.g. Zaragoza) very quickly & were obeyed by most officers & men, so could quickly overpower any opposition. They were supported by many officers who weren't actively involved in the conspiracy, but joined it as soon as conspirators started issuing orders. Quite a lot of senior officers didn't actively take part in the rebellion, but knew about it in advance & did not oppose it. There were very many who refused to rebel, but once the rebels were in control in their area willingly joined them.