How to determine number of devices that can be connected to 3v3 pin of ESP32?
As I can see from the data sheet of the ESP32 it specifies Ioutput* Cumulative IO output current - 1200 mA 
* The chip worked properly after a 24-hour test in ambient temperature at 25 °C, and the IOs in three domains (VDD3P3_RTC,
VDD3P3_CPU, VDD_SDIO) output high logic level to ground.

So as long as your slaves total current consumption doesn't exceed the 1200 mA you should be good, also keep in mind that the wireless transmission will use a lot of that supply.

-- Side note from the data sheet regarding the power supply "its recommended output current is 500 mA or more" .. This is to support said wireless transmission --

As for multiple I2C devices, the bus supports a capacitance of 400 pF, and a max of 127 addresses. That doens't mean you can support the 127, as it all comes down to the capacitance of the bus.