The park entrance passes are for people; the number of vehicles or presence of a trailer doesn't matter... until it's time to pay the separate fee for a campground. The only exception is that the Family/Group pass works for any group of people in the same vehicle (a group in two cars would need two passes).
Parks Canada - Passes and admission
We buy a Discovery pass each year, which works for all national parks and historic sites, for a full year from date of purchase. There are only two of us, but we get one Family/Group pass because it's almost the same price as two adult passes (and so we could take people with us if that came up). Two Senior passes would be a bit cheaper than a Family/Group pass.
If you're not visiting any of the parks for very long, it might be cheaper to pay for daily passes, although that's less convenient. Different national parks have different daily admission rates, but all accept the same annual passes.
Everyone is welcome in Canada's parks (at the same price)... regardless of where you come from.