Afaik, some mods, like at least racemenu presets, (specifically, the simple ones that contain *only jslot files) can be merged as easily as uncompressing each of the source files, then combining them into a single structure (ie, take one of them, rename it slightly, open it, and add the other .jslot files you want represented into it, then save the file using the original compression.) As long as the files are in the right folder in the source file, (I believe) they'll get dumped into the right location so they can easily be called from racemenu. (edit: note, I mean you combine then, then install that mod...so you'd uninstall the others first, to avoid any deployment contentions that *might result in a load order issue downstream)
As an example, suppose I have two racemenu preset files.
Normally, as players, we'd install them as two mods.
The first is an assortment of 16 jslots, the other contains only one jslot.
- open both
- copy the contents of the second into the first
- In this simple case, all you need is the jslot file itself, and you drop it into the same folder where the other 16 are located.
- then resave the first and
- close the second (the second one is no longer needed for this operation, although you might want to save it on its own for archiving purposes or not) .
A potential caveat is that the first 16 were created based on one body type, and the single on another body type preset (ie the bodyslide type, CBBE, 3BBB, or one of the UUNP/etc body type), so at that point you simply need to consider the targets when combining, or at least be aware that the target bodyslides are different for each. (really not all that different from normal, since most racemenu preset mods come with a list of the source material they were made using, and it's often stated what bodytype was used...) (on the other hand, perhaps they're both targeting the same body type...then you're in with Flint, so to speak, ie good to go)
Theoretically, as long as a set of racemenu preset mods are all just jslot files stuffed into the appropriate folder and then compressed, every single one could be combined into a single mod. (In fact, I may just do that myself. I've been trying to cull down the fat.) It wouldn't be distributable of course, unless permissions were granted by source authors, but for personal use...
As to other, more complex merges, idk. I haven't gotten that far in my own efforts to streamline and have no idea what would be required, if in deed it's possible (ie like merging two disparate mods). Some may not be mergable. Some are obviously, once you examine the file contents (ie, like a vast majority of racemenu preset mods) , others, idk.
PS> regarding the limit....I don't need an answer, but I';d ask you to ask yourself... do you *really need all 255+ mods? I asked myself that and found that in reality, no, I didn't. I have some mods (armor clothing, some effects/etc) that I installed to check out and yes, I used for a while, but I've grown tired of them. I cleared about a dozen that way (although I've added about half that back from new mods...) PS> not saying that to discourage yuou from merging mods and reducing your count that way, just maybe you can get even better clearance by being a bit "frugal" about what you really need. You can always archive a source locally if you're afraid it might be gone from nexus next time you decide you want to fiddle with it... Yes, you can play with a mod overloaded setup. No doubt. But at least in my experience, short as it is, skyrim prefers a lower mod count.
edited for content.