The whole question depends on the exact terms of the ticket, fare AND the booking agent ( whether it is a site, or a real agent, or even the company itself.
That being said - and speaking from own ( quit big ) experience in changing and cancelling - The Changes / Cancellation fees in almost always for the TICKET as a whole and not the LEG / SEGMENT. I actually can not remember it ever being different..
But if for example, you will make changes to a single LEG inside of the same ticket - different fees / regulations might apply depending, again, on airline. ( for example not change first leg and make longer stay-over with second )
That being said ( again ) the best way is to contact the airline directly.
A small tip addendum:
Being a business traveler, I travel a lot ( 4-8 flights a month ) I find myself changing / cancelling flights almost 70%-80% of the time. If you are thinking already now about cancellation / changes - or if there is a chance ( even slight ) in you eventually doing so - my own tip and opinion is that it is always better to opt for a higher price ticket with changes permitted. Otherwise in some cases you can simply lose the whole ticket.