In the time it took you to give me the standard "Read the manual" line, you could have answered the question

Maybe I could; however once you get familiar with what info is where, it is a whole lot faster to look it up than to post and wait. as you see in Peters reply, there are many answers to your question, and the whole story (banks, bit addressable, regs) is explained in detail in the text.

A short ansswer "there are eight registers R0-R7" while correct, would not give you the insight into the register structure (complete with pictures which are not possible here) that the .pdf texts give you.

If you look through my responses, you will see that I am not against answering questions directly; however when the full and exact answer is available in a standard text, I prefer to direct the answer there.

Have fun,

Erik