There are two ways to get a monarch: the "normal" one which requires the fortress to be a metropolis (probably 140 pop. Check the wiki), plus a bunch of production targets within the last year (again, the wiki lists those). I've never gotten a monarch this way...
The "abnormal" way is for the mountainhome to get lost and the previous monarch along it (or later). This is typically the case with "struggling" civs, but can happen even if you start with what appears to be a healthy one. In this case a dorf in your fortress takes up the royal mantle ("after a brief discussion...") or, if two years (I think) passes without a monarch there's a different message (with the same effect), but with the difference that it tends to be the expedition leader, which means you have a chance to influence who gets appointed. The "abnormal" route does not bring with it the things the normal one would (royal guard, etc). I've gotten lots of mandate spewers that way.
The third case is a dead civ (actually dead, not just pretending to be: it's bugged, so it's somewhat rare). In that case you'll never get a monarch (rumor has it you might be able to revive a dead civ by founding multiple healthy fortresses, but I haven't heard that it actually is achievable: a single metropolis fulfilling the normal criteria does not suffice, anyway).
80 is the lower limit to get all other features except the monarch.