Hello Hobbit,
It is recommended that WIDS APs be enabled in areas that are prone to attacks from rogue APs, for example, outdoor areas. Generally, the WIDS AP can provide signal coverage for rogue APs and rogue WLAN devices. The WIDS AP does not have a fixed calculation method. You can determine the AP locations where the WIDS function needs to be deployed based on the WLAN network planning diagram to ensure that the signal can cover the rogue devices.
From the following case link, you can see that the deployment of WIDS is irrelevant to the number of APs and the WIDS function is enabled only in areas that are vulnerable to attack.
https://support.huawei.com/hedex/hdx.do?docid=EDOC1100008272&id=dc_wlan_example_sec_0001&lang=en

Hope it is of help to you