Mac Pro sales could easily be 250,000 per quarter.
HP and Apple probably have similar volumes of workstation sales so you could estimate Apple's sales (very very approximately) from HPs.
Of course no one knows what Apple's sales figures are for separate Mac lines as these are not made public.
For another way of looking at it the following survey may give a clue.
If you multiply the survey percentages by the average selling price (Mac Pros are a bit more expensive than Mac Minis...) and you get an estimate of revenue per line...
Forgive me kind Sir, for I have exposed thou eyes to barely comprehensible grammar and abused the English language. I shall attend next possible linguistics class and return when my knowledge and respect for punctuation and uppercases are perfected.