The 3/5/12/24 is correct though with a turret it is actually 1,2,3 based on:
A. the turret type
B. the amount of launchers put on that turret.
For the purpose of most Capital ships though you'll notice that turrets are simply 1 turret with X amount of weapons on them rather then mixed turrets, that section seems for the most part to be in keeping with the older ship design rules.
Each ton of ammunition storage allows for 12 missiles.
The other thing is that a Torpedo Barbette can fire 1 torpedo, a Torpedo Bay at 50 tons can only fire 3 Torpedos at once, a 50 ton bay can fire 12 missiles this tells you that the bay equipment is diffrent. a 100 Ton bay can fire 6 torpedo's and a missile bay 24.. again diffrent equipment.
That can be confirmed on page: 48.
The big thing is that there is NO information listed for Barbettes for the most part, all it says is that it's 1/2 way between a bay and a turret. the indivdual weapon information for Rail guns and Partical weapons give you information but not for existing weapon types..
even the tables on pages 50 and 51 do not list the amount fired.. and the problem is neither actually state what the differance in 'normal' and 'heavy' missiles are.. unless you count heavy as being the newer missile types and 'normal' as the older type.
The other problem is that none of the examples mount missile barbettes so you can't even try and use that as a guestamate of how many missiles they fire.. so presuming all of that I'd say that given the rest is the same as old HG etc it's 3,5,12,24.