I’m very new using Libre, so I asking for help in learning. I started helping a volunteer organization and they have LibreOffice on their laptop. I’ve done the mail merge in Office with Excel and Word to create event tickets for them. But I would love to teach them so they have the ability in the future to do it.
Please edit your question to provide more details. Reopen it (if timeout has not elapsed): click on … below it, then on the “pencil” icon to enter Edit mode (apparently you know how to do it).
Tell us how you proceed in M$ Office. The same procedure may eventually be transposed.
Otherwise, in Writer, I am thinking of using either list numbering (if feasible: the number must be at head of a paragraph) or of inserting the value of a Number range (i.e. a counter). When the number is needed unchanged several times, it is inserted through a cross-reference.
Under MailMerge, you can create the set of numbers in Calc and mail-merge them in Writer.
Mention OS name, exact LO version and save format (stability and reliability are only guaranteed in native format .odt and .ods).