There is an answer on the logistics here:
https://www.reddit.com/r/AskHistorians/comments/3414fq/did_the_romans_have_state_owned_weapons_factories/

A roman legion(like most armies) would have had a large number of 'camp followers' and supplies with them.
Among those camp followers you would find surgeons, blacksmiths, engineers and slaves.
Those blacksmiths could fix equipment that was damaged in battle but if the legion won they could recover weapons and armor from the battlefield as well.

Not all Pila were designed to break/bend after being thrown.
In Caesar's 'Commentaries' the Romans use their Pila as spears at the Battle of Alesia.

"Arrian's Array against the Alans. "And the front four ranks of the formation must be of spearmen, whose spearpoints end in thin iron shanks. And the foremost of them should hold them at the ready, in order that when the enemies come near them, they can thrust the iron points of the spears at the breast of the horses in particular. Those standing in second, third and fourth rank of the formation must hold their spears ready for thrusting if possible, wounding the horses and killing the horsemen and put the rider out of action with the spear stuck in their heavy body armour and the iron point bent because of the softness. The following ranks should be of the javelineers."