Generally, more RAM is better. Monitor / manager nodes for a modest cluster might do fine with 64GB; for a larger cluster with hundreds of OSDs 128GB is a reasonable target. There is a memory target for BlueStore OSDs that defaults to 4GB. Factor in a prudent margin for the operating system and administrative tasks (like monitoring and metrics) as well as increased consumption during recovery: provisioning ~8GB per BlueStore OSD is advised.
берите с расчетом - 6 Гб на HDD, SSD/NVME 8 ГБ. И то, есть вероятность упереться в память при ребалансах, но на это напрямую влияет количество объектов