This doesn't seem to be so much a problem of "how to do the mail merge", since those are pretty straightforward (or can be) once the data is set up properly. But it does seem that you have "a data problem".
You mentioned a table of "about 500 people" who each have "between 1 and 12 roles each". So I'm trying to visualize what your table looks like, and maybe that's where you're also confused. It sounds like someone tried to kinda sorta half-*** the problem by making a table of names, simply enough, and then just adding on roles in the same single worksheet, column by column, growing the thing like Topsy.
With the two primary tables you should have, one of NAMES with unique identifiers for each named person and another with ROLES, also uniquely identified as to RoleID, Role Name, Duties, etc. you can make another table from a one-to-many query of NAMES (as the driver for the table) with a secondary key of ROLES, permitting the one-to-many listing of each name containing as many roles that each name has. That query will be a "make table" query to build the Names and Roles table whenever you need to re-create it. The table will have as many rows as you have names of people, and each person can have, as you note, up to 12 roles. (This may be what you already have in a flat-file worksheet, but you can probably see already how difficult that is becoming to manage systematically. You really want a fully relational database application for this where you can manage NAMES in one table, ROLES in the other table, and using a form to drive the NAMES AND ROLES make-table query in a more organized fashion.)
The mail merge then becomes a process of listing the name (because every row will have at least one Name) and Role1 (assuming everyone has at least one Role), and then using a series of IF statements after that: If Role2 is not blank, then list it and look at Role3 (etc.) and when a blank Roleₙ is encountered, then on to the next field or next record.
You can probably look up how to do that; it's been too many years since the last mail merge I did in that way, but I have done it (maybe a decade ago; that long).
Still all of this is a lot easier to manage in a relational database, though, and you'll want that to manage the list in the first place.