Adding some more details on Salil's comment:
FDD has fixed timing associations between the original data and the re-transmission i.e. it takes 4 ms to get the response for a transmission. In addition, another 4 ms required to send either re-transmission(if received NACK) or new data(if received ACK) based on response received.
As TTI is 1 ms, one HARQ is occupied for 8 ms. What it means is, one HARQ is occupied for atleast 8ms before we can re-use it.
Here, 4 ms = propagation time + processing time
For TDD, as radio frame structure is different for different configuration, we have different number of HARQ processes as per configuration. Though number HARQs for different configurations are calculated with the same logic as above.