RAM recommendations are tricky.  We feel compelled to come up with a formula, but there are multiple factors.  I will share my experience :

We started with min 32 GB per mon, which seemed fine for 12x24 OSDs.  As one specific cluster grew to 41x24 I found that 32 was enough for steady-state runtime, but not enough to boot.  I did emergency upgrades to avert disaster.  

I’ve seen occasions where one mon in any cluster going down caused another, perhaps the new leader but not necessarily, to spike to >100 GB used.   Sometimes a heap release works, sometimes restarts are needed.  

The number of PGs is likely a factor as is the OS - this is on Ubuntu Trusty.  Some time ago there was discussion where Sage expressed a belief that newer releases of TCmalloc behave better.  

Also remember the Ceph-mgr daemon running in parallel.   Usage can grow during distress events too.  

In my situation 128 GB is the min I’ll accept.  YMMV.