Despite the answer being "it depends", I'll ask anyway. How many devices, that take I2S as input, can be connected to a single I2S output? From the I2S protocol I understand there is no problem in doing so.
Currently I'm using an ESP8266 (Wemos) with a PCM5102 DAC. For example I'd like to route the signal also to another ESP8266 or two.
I tried to figure out from the specifications but I have no clue what to look for or how to calculate or estimate.
There is no answer for that. It is a high frequency digital signal. The input impedance is very high. In theory more than hundred, in real life the high frequency can not go through that many wires or cables.
If you put all the modules directly next to each other, then there is no problem.