Hi Rama,
Could we have a Master template with just header and footer and all Templates are derived out of it?
No. There is no concept of 'template' of 'template'.
But you can do that, if you are on 6.2. You can go to template editor and create an AF template. And keep header and footer locked in 'structure' layer, so no author creating form out of that template will able to modify it.
Also if you update the template header and footer, all forms created out of that template, will show updated header and footer.
https://helpx.adobe.com/aem-forms/6-2/template-editor.html
Typically in implementations how the number of templates is decided?
It depends on how many forms share same content, in same order.
In my case, since all 90 forms are different, except in header and footer, is it reasonable to assume 90 templates?
Why will you create 90 templates. You will just need to create one template with locked header and footer. And create 90 forms from this template.
Can you elaborate on your use case?