If you purchased an office key either with email delivery or keycard delivery and then activated that key, then open your Microsoft account and go to 'Services & subscriptions'. Next to each product, click 'View product key' to see full keys. The bottom most keys are the most recently registered. I suggest keeping a spreadsheet containing the user, purchased key, the activation date and the new key (if a new one is issued)
To find the last 5 characters of the office products installed on your PC, open the command prompt and enter one of the following:
(If you have 32bit office on 64 bit windows): cscript "C:\Program Files (x86)\Microsoft Office\Office16\OSPP.VBS" /dstatus
(If you have 64bit office on 64 bit windows): cscript "C:\Program Files\Microsoft Office\Office16\OSPP.VBS" /dstatus
(If you have 32bit office on 32 bit windows): cscript "C:\Program Files\Microsoft Office\Office16\OSPP.VBS" /dstatus
To remove a key found above, enter the following:
(If you have 32bit office on 64 bit windows): cscript "C:\Program Files (x86)\Microsoft Office\Office16\OSPP.VBS" /unpkey:<LAST 5 CHARACTERS>
(If you have 64bit office on 64 bit windows): cscript "C:\Program Files\Microsoft Office\Office16\OSPP.VBS" /unpkey:<LAST 5 CHARACTERS>
(If you have 32bit office on 32 bit windows): cscript "C:\Program Files\Microsoft Office\Office16\OSPP.VBS"/unpkey:<LAST 5 CHARACTERS>
To enter a new key, enter the following:
(If you have 32bit office on 64 bit windows): cscript "C:\Program Files (x86)\Microsoft Office\Office16\OSPP.VBS" /inpkey:XXX
(If you have 64bit office on 64 bit windows): cscript "C:\Program Files\Microsoft Office\Office16\OSPP.VBS" /inpkey:XXX
(If you have 32bit office on 32 bit windows): cscript "C:\Program Files\Microsoft Office\Office16\OSPP.VBS" /inpkey:XXX