Crossbow
The Crossbow is a long awaited weapon for the Assassin's Creed series. This weapon was originally seen in a trailer/intro for the first Assassin's Creed game, but was not included due to historical inaccuracies. However, the crossbow in Brotherhood has the same design as the original in the Assassin's Creed trailer. The crossbow has been confirmed as a weapon in Assassin's Creed Brotherhood. After being spotted in concept arts, and being mentioned in the E3 2010 Single-Player walkthrough, Jean Francois Boivin stated that long ranged weapons such as the crossbow will be available to be used in the game, especially in advanced horse combat. Some guard archetypes will have the crossbow as a weapon in Brotherhood. Ezio is able to use the crossbow in melee combat, attacking guards with the metal lathe and then shooting them once they're on the ground. In the third developer diary it was stated that the crossbow would be upgradeable, what you can do to it are unknown.
Ammo
The max amount of ammo is suspected to be 25 bolts. Ammo is supposedly purchasable from blacksmiths if the economic system remains the same as it was in Assassin's Creed II.