ECash uses Bitcoin. Bitcoin has a fixed supply.
Discussion
Ecash can trivially be fashioned to work with any crypto (send mint crypto, dispense tokens). It's not exclusive to Bitcoin. There is already a Bitcoiner developing Monero Ecash and a bounty for it.
Doesn't matter. A mint can covertly print as many tokens as they want. Bitcoins fixed supply doesn't limit that in any way.
Malicious mints aside. The mints who want to last long-term uphold Bitcoin’s fixed supply.
if you're really able to believe such a ridiculous idea than what use is proof of work at all?
Oh noooo .01% of my stack goes in a trusted ECash mint that offers instant asynchronous fully anon payments boooo. Worry about your own money
hOw dArE yOu UsE a BaNk 
people are suggesting for DNMs with millions in volume to do their business inside ruggabe ecash mints, and that ecash mints will eclipse monero completely. please stop pretending this advocacy is confined to pocket change. and we both know the whole point of L2 scaling is to inherit security from L1 and execute transactions somewhere else. you toss the security from L1 into the garbage bin the moment you put one toe into an ecash mint. the developers are retarded and need to go back and come up with something better.
Things like ECash (and Lightning) ruin the fixed supply of Bitcoin.