You already need a tanker per rocket. How else are you going to get enough throughput on fuel lifting?

You can do that, but it would be doing it the hard way.

The easiest way is the one laid out by Musk -- launch the MCTs into orbit, then refuel them in the 18-20 month "lull" between Mars transit opportunities. Otherwise you wind up building a lot of unnecessary tankers, BFRs, and launch sites just to sit idle 90% of the time. Better to have a smaller number of each and bump up the utilization.

I also think that you can build tankers and keep them around to keep holding fuel as you build more colony ships, because the colony ships probably cost more.

The colony ships cost the same, whether they're refueled incrementally over months or all-at-once. The only thing that changes is how many tankers you need (fewer is cheaper).

I've been trying to find a way to improve on Musk's overall architecture, but so far I've come up empty-handed. It really is very carefully thought out.