That’s a GTA V limitation, not a FiveM one. You can find more info here Question about clothing slots and here YMT game limit and crash issues · DurtyFree/durty-cloth-tool Wiki · GitHub
Sadly, R* most likely won’t increase their limit until it’s needed for Online (like they did in b2612).
I remember there being a discussion about increasing the limit through fivem, and how it’s more complicated than it looks, but I can’t find it on the forums so it might have been in a chat.
Anyway, as the github post suggest, you might want to start replacing existing props, if you’re not satisfied with the addon limit.