Q1. Yes, you could redeem up to 25 reatil one-time purchases of Office 2019 on the same Microsoft account. For more, please refer to "Microsoft account limits for Office".
Q2. For the senario that more than 25 licenses, Microsoft recommends using the volume-licensed version of Office. So that you do not need to redeem the key to activate Office.
It seems to be inconvenient to bind each of hundreds of licenses with a different account.
If you redeem multiple one-time-purchase Office with a single account, it's best to refer to the artcile "Manage multiple one-time-purchase Office installs that use the same Microsoft account". You should make a list that includes the machine information (Such as computer name, S/N number), key, account.
Q3. >> instead it showing same different key their
<< Accoring to the Step 1 in article above, for each Office product that's listed on the Services & subscriptions page, choose View product key you may find the related information. Do you mean the key information you find is a same one?
Q4. As the Step 2 in article above, Microsoft shows the last 5 characters of the product key on your machine. You may run cscript "C:\Program Files\Microsoft Office\Office16\OSPP.VBS" /dstatus
to get it.
Q5. Once the license is activated on a Microsoft account, it's unable to transfer it to another microsoft account. Here is a similar thread you may have a look.
"New Office 2019 Home & Business transfer to another Micrsoft account. Just activated by mistake".
Q6. I would suggest you contact Microsoft team online to seek help. (Please remember to get round the Virtual Agent, and then find the human service).
Hope the information could be helpful.
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