Easy formula. Take the Holding M3 of the Assassin and add up the ships/Vehicles (your adding to the docking bay) from their weight/Ton. If your looking for RMs, that is based on the material its self.
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What do you mean "it doesn't help" ?
The rules pages have the weight and volume capacities of all ships.
Just compare those of the Assassin class to those of any other ship.
Weight: 13,500 T
Volume: 275,000.0 m³ Weight Cap: 2,500 T
Volume Cap: 15,000 m³
The Bold shows what the capital ship can hold
The nonbold shows the capital ships actualy weight and volume... just compare the fighter's weight and volume to the cap's capacity to find out how many will fit