I changed up the Serverconfig of making to cauldron bigger to true ( cauldron {
# Makes the cauldron hold 4 bottle per bucket instead of 3. Translates better to modded fluids. [default: false]
B:bigger=true ) and on my Server if I now place a cauldron its still 3 bottle per bucket can you help me with that?

You need to do it on both the client and the server, just setting it on the server won't work.

The config option works, it was a big part of SkyFactory and I received no reports of it not working there, so its something on your end. 1.12 is also not supported anymore.

One possibility is you have another mod doing a registry substitution, e.g. I know there is a mod that improves the cauldron hitbox that conflicts with Inspiratons; that feature is in Inspirations already.