when Octavian marched on Rome for the first time (he did it on several occasions, hehe, came to his great-uncle) he had an army composed of 1) his own which he had recruited by himself 2) the armies of Hirtius and Pansa. Some eight (?) legions - I can look for the precise number.

At Rome a couple of further legions deserted to him. After being "elected" as consul he and an army of some ten legions went to face Antonius and Lepidus in Cisalpina.

Well, he could afford it. Firstly, he had Caesar´s treasury for the eastern campaign. Secondly, at Rome he could steal whatever left from the treasury.