By powering the esp8266 using usb, I can power a maximum of 6 such modules. However, I need to power as many as 20, and I don't know how it would be possible to power 20 boards at once with one cable.
What exactly are the esp8266 modules ? if it is a D1 Mini, you have a 5V pin and supply power to it from an external power supply and not from USB. You need to anticipate about 0.3A per module, as this is how much it can take in peak for WiFi. So if you have 20 modules, you will need a power supply of at least 6A 5V.
