It is not a stupid question, but it is one that can be answered by checking out the BattleTech rules (an abbreviated version of the rules can be found via Catalyst themselves here:
http://bg.battletech.com/?page_id=27)
Missile launchers in BattleTech--including LRMs, SRM, streak SRMs, ATM, and MRMs--fire swarms of missiles. An LRM 15 fires 15 missiles in a single salvo. Each missile does a small amount of damage (1 per missile in the case of LRMs), not all missiles in a salvo might hit, and those that do will be scattered all over the target.
To make bookkeeping easier, the ammo count on the record sheet does not count the total number of missiles, but the number of salvos. Ammunition in BattleTech is allocated by ton (with only minor exceptions), with one ton of ammo occupying one critical slot.
So: the Trebuchet carries 2 tons of ammunition. Each ton consists of 8 "salvos", for a grand total of 16 salvos of 15 missiles each. Each time a weapon fires, it expends one round--or salvo--of ammunition. As the Trebuchet has two LRM-15s, it can expend one shot per launcher fired. If it were to fire both launchers in a turn, it would expend a grand total of 2 rounds of ammo (one for each launcher) and send a total of 30 missiles down range.
All ammunition weapons in BattleTech consume ammo this way--the number on the sheet is how many rounds in that ton, each shot expends one round. Some weapons (ultra autocannons, rotary autocannons) can fire multiple shots in a single game turn, but they are the exception.
Edited by dashukta, 22 August 2012 - 10:49 AM.