Hannibal arrived in Italy with around 35,000 infantry and cavalry. This hardened force served as the core of the army he built in Italy, first with Gallic recruits who hoped to throw off Roman rule and then by recruits from Italian cities which defected to his rule. By the time he faced his first serious Roman opposition in the Battle of Ticinus, his total force was close to 90,000.

This shrank to around 50,000 by the time of the Battle of Cannae, as Hannibal detached garrisons and left the lands of his new Gallic allies. (By this time, he'd also lost the remaining few elephants he'd brought.)

After that victory, Hannibal continued fighting in Italy for 15 years, never able to raise a large enough force to threaten Rome itself. In 207 BC, a decade after his victory at Cannae, Hannibal received word that a long-awaited relief army led by his brother Hasdrubal had been crushed before reaching Italy. After this loss, Hannibal was steadily pushed out of the cities he'd brought over to his cause. When he finally left Italy in 203 BC in order to attempt a relief of the Carthaginian forces in Africa, he had 20,000 soldiers left.

For details, I recommend The Punic Wars by Adrian Goldsworthy, The Ghosts of Cannae: Hannibal and the Darkest Hour of the Roman Republic by Robert L. O'Connell, and Hannibal's Dynasty: Power and Politics in the Western Mediterranean, 247-183 by Dexter Hoyos.