Can RX pin(s) be used as TX instead?
One way to communicate Arduino to Arduino device is UART (and Software Serial for additional pins). For the ESP32, it seems this allows 28 TX pins using Majenko's method (in their answer). In other words, slave 28 devices.
After further research, isn't I2C the clear route to go?


With more slave devices the less times per second to talk to them all
The application needs to have all the slave devices execute commands synchronously:

At least, the slave devices need to start functioning at the same time.





How many slave Uno's/Nano's can be connected to an ESP32?


Each device on an i2c network has a 7-bit address, so a single network theoretically supports up to 128 slave devices. In practice though, the limit is much lower. i2c slave chips often support only 8 different bus addresses, no more than 8 of that chip can be attached to the same i2c network.

https://mindbleach.com/words/2012/10/23/i2c-and-device-limits
Unclear answer:

https://forum.arduino.cc/index.php?topic=159860.0
https://forum.arduino.cc/index.php?topic=367961.0