One check on a magistrate's power was his collegiality. Each magisterial office would be held concurrently by at least two people. Another such check was provocatio. Provocatio was a primordial form of due process. It was a precursor to habeas corpus. If any magistrate tried to use the powers of the state against a citizen, that citizen could appeal the decision of the magistrate to a tribune. In addition, once a magistrate's one-year term of office expired, he would have to wait ten years before serving in that office again. This created problems for some consuls and praetors, and these magistrates would occasionally have their imperium extended. In effect, they would retain the powers of the office (as a promagistrate), without officially holding that office.
How many people would serve concurrently in a single magisterial office?
at least two people