Well, originally there where 7 (well, 6 really as frame pointers were pretty much required) general purpose registers. AMD64 bumped that to 14-15. That's a huge change with clear performance benefits. going to 32 is already into diminishing results for most use cases and has non-trivial encoding, function calling and context-switching implications.
Shared libs on the 386 stole a register more, because IP-relative addressing wasn't a thing until AMD64 (except for jmp/call). Shared libs need to refer to their own data + they may need to form pointers to code within them.
(Shared libraries need to have a pointer to their code and data. This is done by creating a small table of entry code in front of the "real" code that sets a register to point to the library code/data memory before jumping to the real code. This code will look different for each process that has loaded the shared library so it gets its own little memory block. The rest of the code/data will be the same in all processes so it can be shared between them -- unless/until some of the data is written to, of course.)
So code in shared libs really only had 5 registers unless the frame pointer was fomitted. That's why it was necessary to tell the compiler whether to build normal code or shared library code.
The AMD64 not only had more registers, it also has an RIP-relative addressing mode so it doesn't need to steal that register for shared libraries.
