The soft limit for mods in FNV is 140. If someone is having errors underneath this amount, it's because they're inexperienced and have conflicting mods and load issue errors. They might also have either out of date dependencies, or outright lack them entirely. Also, it probably doesn't help if you have an extremely old processor/gpu and are running several complex mods all together. Personally, I have no issues with my load orders in FNV, but I use MO, and have a dedicated safe load order with 120 mods, whilst I play around with higher amounts on other profiles.
Unfortunately, there is no soft limit in Fallout New Vegas because the limit is supposed to be 255, but the engine is broken in that regard, and without the source code we can't easily find the problem, so we can't fix it.
The most common "soft limit" is around 125 and still a vast majority of people have "too large LO" problems before that.
Like I said before, some people have problems with 100+, some only have problems with more and some have problems with less (this last one is less frequent). Sometimes you can even have a limit of, let's say, 110 in one playthrough, then you uninstall the game. Later you reinstall and the limit might be a totally different number.
Also, even if you have plugins that are not active in your Load Order, they will count for the limit if they are left in your Data folder. Because the game still indexes them. Even if they are not activated in your LO. So always remove the inactive plugins from your Data folder (some Mod Managers prevent this by not installing plugins in the Data folder though).
And let me repeat myself, be careful with large load orders (100+), because many times the problems only start appearing after dozens of hours of playing, the longer you play that game, the more problems will appear and it will usually become baked in your save files too. So even if you reduce the LO and load the save file, it will still be broken.