Depending on the configuration, unless the 120 sites share the same webroot, generally speaking Lucee will run each site as a separate context which requires more memory than ACF.
Lucee 6 will support a single context like ACF, I seriously doubt changing threads config will help you here, you are simply running out of memory required for all the contexts.
5.3.8 does at times use less memory than 5.3.7