There is not necessarily a set minimum number of participants that we would recommend to take the IDI in order to get a clear picture of where your organization is at. It really depends on your goals. Because an organization or group is made up of people, to get the most accurate picture of the organization, ideally you would administer it to as many individuals as possible. However, we understand that this is not always possible due to lack of resources.
Thus, one option is to start with all members of leadership teams, if that is an option. This would not be representative of the organization as a whole, but would represent those who likely have the most influence, make policies, etc., and can provide some useful information.
If you opted to do a sample of the organization, I would encourage you to think carefully about how you select the individuals who are asked to/allowed to participate, and what implications that might have on others in the organization who do not get the same opportunity.