Formula to produce sequential number for multiple suppliers

Hi there,
I am trying to put a formula into my spreadsheet so that each saw mill gets a sequential number added automatically so that we can track the number of loads we have taken from each saw mill. I am wanting to have saw mill Bob's numbers to follow on in order but may have an entry for another saw mill in amongst the list of collections
Can anyone help please? I have attached a sample of what I am trying to do.