Supply Reserves determines how well military buldings support your forces and garrisons. If Supply Reserves is negative your armies will consume additional supply. Otherwise you will receive a small bonus to unit recruitment cost and garrison buildings cost.

It is optional feature and you can activate it in MCM. After this a new line will appear in treasury tooltip.

It doesn't work with wood elves factions

From military buildings(red and blue)*
Tier 1: +1
Tier 2: +2
Tier 3: +3
Tier 4: +3**
Tier 5: +3**

* - Buildings in Balck Arks and Vampire coast ships don't count
** - you can increace this value in MCM

From city level
Level 1-3: -1
Level 4: -2
Level 5: -3

From any garrison building: -1

From armies*
1st army: 0
2nd army: -1
3rd army: -2 (-3 total)
4th army: -3 (-6 total)
5th army: -4 (-10 total)
...etc**

* - Gotrek's army and Black Arks doesn't count
** - you can set maximum for supply reserves per army in MCM

So if you have province with 4 cities, 5-tier capital and garrisons in every city, your supply reserves will decrease by 10. To compensate for this, you must construct 4 military buildings (+12 if they are at least tier 3)

• I don't recommend enable supply reserves in middle of the campaign. You can but you won't like it. Anyway you can disalble it at any time.
  
• This feature works similar to "Global Unit Cap" mod. I don't recommend using them together
  
• It is ok to have negative supply reserves in early game. 2-3% additional upkeep is nothing
  
• Ther is new tooltip then you raise new lord. It will help you assess how much will change supply reserves after this action
  
• If you play with mods which add new military buildings and doesnt include in list of supported mods please write in comments
  
• If you have any suggestions, questions or problems with supply reserves write it in comments