The way the Roman senate positions works is that from the various senatorial class families some run for election or selection to the various public offices. Although in RTW they are nurfed some what in the real roman republic there were 2 consols, 6 praetors, quaestors, and adiles 1 pontificus maximus and censor. Unlike a modern senate the Roman senators weren't elected the plebs elected people to the postions I listed from the senators and other patrician classes (patrician=nobility). By the time Caesar was assassinated there were about 800 senators, 300 were senators by Caesar's appointment.