Nothing really. The CPU that was most influential in kicking this whole thing off was 8086. IBM PC and co, so the number became valuable and they followed it up with 80186, 80286, 80386 and thus 80x86 pattern which got shortened to x86.
8086 was from Intel's naming scheme at the time. Its been a while and I am sure there is a guide somewhere in the Internet but from what I recall:
1st digit was about technology, I believe it started with PMOS, NMOS etc but the ones interesting to us are 4 and 8 which denote 4-bit and 8-bit chips (at least initially, since 8086 is 16-bit).
2nd digit was chip type. 0 processor, 1 RAM, 2 controller, 3 ROM etc.
Last 2 were generally sequence but sometimes pretty freeform. Not all numbers got to be a product and it was not always a sequence. "Sounds nice" was sometimes a deciding factor as well.