Datasheet register bit number
It is common for register bit numbering to start at bit 0, but there are exceptions. That is why, to be sure of the correct answer for a specific device, you need to read its datasheet:


If you see a mention of bit 8 within a byte, then the bit numbering starts from 1
If you see a mention of bit 0 within a byte, then the bit numbering starts from 0


Obviously things are more complicated when referring to registers larger than a byte (i.e. larger than 8 bits), but the same principle applies.

Here is an example from a datasheet which you've asked about before:


[Source]

You can see that in this device's datasheet, a register's bit numbering is from 0 to 7 as highlighted with the red border, so that answers your question about the starting bit number.