Too Many Masters

[Noggog]

Often, after adding a patcher, or rerunning Synthesis after adding mods, it fails with the following error:
System.ArgumentException: Synthesis.esp has too many masters on masters list. ___ >= 254

[Noggog]

Problem

Consider you have 1000 mods on your load order.

Mutagen/Synthesis is able to read from all 1000 when creating your custom patch. However, if your patch decides to override records in each of those mods, then it will have 1000 masters. When this patch goes to write to disk, this will fail, because Bethesda's mod format has a hard limit of ~254 masters that a single mod can reference.

So generally, patchers can consider any number of mods, but they still cannot modify more than ~254 at once.

But it happens. People have a lot of mods. Some patchers do a lot of things.
What can be done?

Split your patchers into separate groups

https://github.com/Mutagen-Modding/Synthesis/wiki/Multiple-Output-Targets

With less patchers funneling into each patch file, there's less chance they go over the limit.

This of course requires you to do a bit of balancing yourself to find a desirable setup. You could put each patcher in its own group, but then you'd likely have more patch mods than needed. If you put all your patchers into one group, then you again risk too many masters.

Patchers can offer settings

Some patchers that do a lot of things sometimes offer settings. Patch Npcs (yes/no), Patch Weapons or other similar controls. If the patchers give you these types of settings, you can then begin to split a single patcher into several groups. Put the same patcher on twice in different groups, and turn Npcs on for one, and Weapons on for the other. Now each resulting patch will have less content and be more likely to be under the limit.

Merge Mods

One alternative is that you can start to compress your LO to have less mods. ZMerge and other tools can merge several related mods into one, so that downstream patches can import less mods overall.

Cry and Wait for More Features

If none of the above options work for you or are available, then you might be out of luck. A patcher very well may just try to do too much and go over the limit no matter what you do.

There are some future experimental features in mind that will overcome the maximum amount of masters, but there's no timeline for when those might happen.
Mutagen-Modding/Mutagen#144