There are 4-8 entries of Thnderbird.exe in task manager. The first is 400-800mb, others are 50-150mb

> What can I do?

Difficult to say. But https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems is a startign point

Tried all steps. How many processes of Thunderbird.exe should there be ideally and how do I reduce them? And how large should the combined total or a single entry be?

Tried all steps.

Then there is likely nothing further to be gained. If you are not experiencing performance issues I would not invest further time.

How many processes of Thunderbird.exe should there be ideally and how do I reduce them?

Version 91 will have at least 3-4. It cannot be reduced.

If you have 8, then somehow you have Thunderbird running multiple times, or multiple 3-pane windows.

And how large should the combined total or a single entry be? (The first is 400-800mb)

You should expect one to be in the nominal range 400-800mb which is certainly within average range. But some users my see up to 1.5gb depending on their profile size.

RE Performance issues: I do get delays with clicks from one folder to another and change of clicking to change folder views (unread, favorite, unified, etc). Everyday, I have a period where the app just freezes, folders cannot be clicked on and my cursor becomes an hourglass symbol toggles to an hourglass icon every 2 seconds for 15-40minutes. My profile before updating from version 68 was 3.66gb, and is now 4.42gb and growing.