For an example of what the limit can look like, we can go to my addon3 folder and take a look around.
Let's see...on first glance, pretty empty:
AAA-v1.3patch
BDT
BF1
ETR
MWP
RPH
SK1

but that almost fills up the mission count, because:

BDT is Dark Times
BF1 is the Convopack
ETR is Emperor's Throne Room (this could go in my addon1 or addon2 actually; it's a standalone map)
MWP is Mandalorian Wars Project
RPH is a top secret (kind of) beta of RAS Prosecutor: Hangars--put in addon3 because it's "special"
SK1 is KotOR era missions for that custom GC.

So, basically 4 largish (mostly era) mods fill up the mission count.

The 2.2 updated for the Convo pack reduces the pack's mission count. Did you install that? If not, do so.

Thanks for the 000 tip, though I'll have to recheck if the game loads numbers before letters or not. I can't say if I'll ever finish v1.4 right now. I'm having a bunch of life issues at the moment and am even moving next week (so my website will be down for at least a week if not more).