And the whole argument gets muddied by the introduction of the ESL format. (A good thing from the Creation Club.) Both LE and SE use an 8 digit hexidecimal number for the FormID (i.e. x'2E0058CF'), with the first 2 digits representing the 'slot' number, allowing for 256 slots. The first slot, x'00', and the last, x'FF', are reserved (first for Skyrim.esm and last for system use only.) The 6 remaining digits are used to identify the form/object/resource/whatever within the plugin. This means that LE only has the 2 hexidecimal digits to distinguish between plugin files, so taking reserved slots and DLCs into account, you can add 250 esm/esp files to your Skyrim Legendary Edition. In SE, with the introduction of the Creation Club, Bethesda added the ESL file format. This changed the use of slot x'FE', reserving it for ESL plugins, and changed the use of the digits within the FormID to define a 3 digit 'ESL Load Order' within that slot. I.E. the FormID for ESLs is defined like x'FExxxyyy', where x'FE' is the slot, xxx is the ESL load order and yyy is the resource id set by the plugin. Halving the number of digits used to reference the resources does limit what type of mods can use the ESL format but the trade off is that you can have, theorectically, 4096 ESL plugins. (There are apparently other factors which could limit the number of ESL plugins to about 3000 or so.) So the definitive answer is 249, cause SE comes with the DLC. So total of 256 slots, -1 for Skyrim.esm, -4 for the 'DLC', -1 for ESLs and -1 for system use only, leaves you with 249 slots to load esp files. (Which could be merged files if you want.) Or it could be 4345 if you want include the maximum number of ESL files (which could be esl flagged ESPs). Or you could add an unlimited number of mods if you want to include texture only mods that don't have an ESM, ESL or ESP...
