too many people don't realize just how non-portable react code can be.
My previous company had probably well over a dozen web engineers and then for our mobile side we had a total of 3, myself included. We also often had to adapt designs ourselves and work with product to fix flows for native. But we still managed to keep up with web.
edit: and not "keep up with web" in the we are working 80 hour weeks type thing, very normal working hours, 9-5. Possibly more normal than the web team.
So in a lot of cases having just 1 engineer per each mobile platform would be enough. And if you spend say 3 years building backend + web frontent, it would take about 6 months for each to replicate user facing experience on mobile. With current average global quality engineering cost of 70k/year, this means it only costs about $70k total to ship both mobile apps.
There are many ways to do it less efficiently, but this is my personal experience (as a CEO, EM and also iOS engineer in different setups) and it's really achievable.