How to use more than 2 I2S microphones on MKR Zero?
By definition, the I2S bus only allows up to 2 channels to be transmitted at the same time.  Here is a quote from the Details section of the Wikipedia's I2S page:


  The word select clock lets the device know whether channel 0 or
  channel 1 is currently being sent, because I²S allows two channels to
  be sent on the same data line.


This is further illustrated in Figure 8 / Page 6 of the Knowles SPH0645LM4H data specifications.  In this figure we see where 2 SPH0645LM4H microphones are configured (ground or Vcc to the SEL pin 2) such that they can be multiplexed onto the I2S bus by the I2S bus master.