The game engine uses the first byte (the load order form ID) of the 4 byte form ID to determine the plugin load order. A byte goes from 0 to 256, 0x00 to 0xFF hexadecimal. The game uses the load order form ID 0xFF for save game and temporary data.
Skyrim, Skyrim VR and Enderal have a total limit of 255 (0x00 to 0xFE) full plugins (ESM + ESP). They do not support light plugins (ESL) - ESL flagged plugins with *.ESM or *.ESP filename behave like full plugins.
Skyrim Special Edition, Skyrim Anniversary Edition and Enderal SE have a total limit of 254 (0x00 to 0xFD) full plugins (ESM + ESP) and 4096 light plugins (ESL), which use 0xFE as the load order form ID.
Mod managers like MO2 (hover over the number of total active plugins top right on the Plugins tab) or Vortex (top right on the Plugins tab) can show the total number of ESM+ESP and ESL plugins.
Make sure to install SSE Engine fixes to increase the maximum number of file handles (MaxStdio setting in the EngineFixes.toml) to avoid the false save corruption bug and other problems caused by the default value of 512. Note that the game keeps more file types open and not only plugins, for example logfiles.
The game will start with too many plugins without any error messages and it might even work for a while before crashing or exhibiting severe errors.
In case the plugin limits are surpassed when generating LOD, execution of DynDOLOD will stop with a too many full modules message.
In case the plugin limit is surpassed in the game, there might be an error message "DynDOLOD.esp not found or too high in the load order". The game will load too many plugins and may seem to work for a while until exhibiting strange errors and CTD, saves will have unrecoverable problems.
Disable, merge or in case of Skyrim Special Edition convert full plugins to light plugins if possible to reduce the number of full plugins.
Remember that there also needs to be room for adding one or two DynDOLOD plugins and maybe the Occlusion.esp.