I'm expecting that SpaceX will have lots of towers, not just one (currently) or 3 or 4 (under way). It won't just be for redundancy. The duty cycle of a tower might simply not allow for the cadence that SpaceX wants to maintain. With Falcon 9 they currently have a 3x weekly launch cadence (which is unbelievable enough). With full reusability they might be able to get to daily and better cadence, so if the duty cycle of one tower does not support that (I assume right now it does not) then they'll need more towers.
A flight to Mars is currently going to require 10 tanker flights just to get enough fuel into orbit for 1 Starship trip. Containing liquid fuel for long in orbit will not be easy.
This likely means they are targeting 10 flights in a day at least. They've mentioned 1000 trips to Mars during one transit window, which means ~10000 Earth launches within 3 months, or >100 per day.
