AMD64 why only 8 new GPR and not 16 or 32?

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x86 is known to be register-starved, with only 8 GPR

AMD64 extends x86 8 GPR's to 16GPR. Many RISC such as PowerpC uses 32 GPR, which makes me wonder why AMD didn't extend the registers to 32 or even 64 total GPR
(same can be said for SSE, from 8 to 16, why not 32 or 64?) Itanium has over 100
 
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I thought AMD64 ISA under "long mode" with register extensions has a GPR width of 64.

As you prolly know the Itanium is a specialized processor that in itself can cost several hundred dollars.