Forge 1.20.1 latest version.

Since brewing stands can contain a single potion per slot, connected comparators should give out the usual (vanilla MC) redstone signal of 3 when 1 potion is present, 6 with 2 and 9 with 3 no matter the stack size you choose in the configs.
Now that they are 16-stackables by default comparators give out power 1 no matter how many bottles there are inside.