I'm on Level 18 with the 13th expansion in progress. It was the only one offered, whereas we've had a choice between 2 plots from the 7th to 12th expansions. Supposedly, the 13th is the last for now, whichever plot you left for last. In my case it's the Pumice plot in that uppermost corner of the first cliff.
After this, I guess I'll start tackling the most costly obstacles until they unlock more expansions.
I know of someone whose runes are maxed out in storage and has nothing to expand.