How many external devices can be linked to a CPU with n address buses and m data buses?
You have correctly calculated that with 8 address lines and 8 data lines you can uniquely address 2048 bits.

However, your question is vague in that it uses the word register without any explanation of how many bits are in a register. You must know that to answer the question. As DaveTweed pointed out, your conclusion is only valid if each register is just one bit. If the registers have more bits than one then your answer would be different.