AVR registers location
No. The CPU registers and I/O registers are in their own set of locations independent of the on-board SRAM. They can be accessed via normal memory access instructions, but they do not take away from the size of available SRAM.

For illustration, here is the data memory map of the ATmega128A, a AVR device with a full complement of CPU registers, 224 I/O register locations, 4kiB of SRAM, and up to 64kiB of external memory space. Note that all 4kiB of the SRAM is both visible and accessible regardless of the number of registers.