The Roman senate started advising,
went to ruling and ended applauding
Numbers of the senators went up, up and down
Romulus instituted the senate as an advisory council with 100 members. According to Livius, Tullus Hostilius was the first to expand the senate with the nobles from the destroyed Alba Longa and Tarquinius Priscus expanded the Senate with another 100.
Tarquinius Superbus had a lot of senators, who wouldn't agree with him, killed and not replaced. After Tarquinius Superbus had been expelled by L. Junius Brutus:
The senate had been thinned by the murderous cruelty of Tarquinius, and (Junius) Brutus' next care was to strengthen its influence by selecting some of the leading men of equestrian rank to fill the vacancies; by this means he brought it up to the old number of three hundred. The new members were known as "conscripti," the old ones retained their designation of "patres."
Changing role and power
In the time of the kings the senate was nothing more than an advising counsel. In the time of the republic the institution would gradually gain power: Laws had to be approved after the the comitia centuriata had done so. In the time of the Punic wars it was the senate that decided which consul or proconsul had to to to which war theatre and what means he would get. See Polybius (6.13).
Abguss museum München (GE)
The man who would be king
Though Caesar increased the number of senators he decreased their power and gave them the impression he tried to become king, because he had himself appointed as dictator for life. This would lead to his murder.
In power reduced
His adoptive son, Octavianus, played the game better and gave the senators the impression they did matter, but reduced their power to an advising role. Augustus.
At the end of the third century Emperor Diocletianus would reduce their influence to zero.