It sounds like you could consider your project to be a set of comparative case studies. If so, then I would recommend Yin's book, Case Study Research. His approach is to develop a "protocol" that you apply to each case, so that you have comparable data from all of them. This would not necessarily require you to interview a full set of stakeholders from each site, as long as you felt like you had high quality information for each of the crucial topics at each site.

It sounds like you could consider your project to be a set of comparative case studies. If so, then I would recommend Yin's book, Case Study Research. His approach is to develop a "protocol" that you apply to each case, so that you have comparable data from all of them. This would not necessarily require you to interview a full set of stakeholders from each site, as long as you felt like you had high quality information for each of the crucial topics at each site.

You don't have to necessarily interview them one by one but if you could it would be beneficial for your study. Prepare a questionnaire addressing the crucial information they may have and send to them. Make it brief and practical. There are many guidelines here and there on the internet about how to set a good questionnaire.

I agree with the above responses and would add that you consider a screening survey before attempting to complete 90 interviews. We've used screening surveys to collect a standard foundational dataset from all stakeholders in a construction projects and to find out who is actually willing to be interviewed.

I wish you the best of luck because my construction research experience has shown that it is easier to collect "good news" data than "bad news" data from project stakeholders.

l agree with the above response and would advise you check your focus so that your case study will be spefically as intended, rather than a generalised one. hope this helps.