There are very few mods for Org Sins and I don't recall a current one for fleet supply.

Always good to have a new modder in the community and everyone here is extremely helpful if you want to learn.

A few points

Always show what version of what expansion (are you up-to-date patching org Sins?)

Unlike diplomacy and entrenchment which use a manifest system, org sins requires the mod to have the complete set of reference files.

The reference files supplies by Iron Clad are very out of date. Do you have a current set or convert your own? 

I created a weebly with a lot of articles by the community and myself that can help you out.

Modding Home	Modeling Home
Placing Mods	Modeling Tools
Placing Maps	Texturing
Modding Tools	Getting Rid of Non-Tris and Non Quads
Crap, bad link	Creating the UV Map
Developer EXEs	Hooking-up Textures to the Model
Hard Code Limits	Creating the Tangent Map
Sound	Setting-up Meshpoints
Spiral Galaxy Fix	Cleaning-up Your Model
Add Planets	Exporting your Finalized Model
Add Race	Creating a Second Set of Materials
Research Button	Import Models
Particle Forge 3	Shared Models
Convert Data	Convert XSI

Download Update Validate

RESEARCHSUBJECT_MAXSHIPSLOTS

researchModifier
    modifierType "ShipMaxSlots"
    baseValue 100.000000
    perLevelValue xxx.000000

Change all for the races in whatever mod you have or Vanilla if you want incompatibility issues (don't mess with vanilla)

You can get crazy fleet supply. 

Other way is to change scalar in constants for fleet size as Lavo_2 mentions, which is the easy way to do it.