To fully evacuate a vSAN host and satisfy FTT=1, FTM=RAID1 you must have at least 4 hosts in the cluster. When a host is put in maintenance mode and fully evacuated, that host data is spread across the surviving hosts. In other words, if you follow the vSAN best practice guidance to stay less than or equal to 70% utilized, then the capacity that represents the 70% utilization must now fit on 3 hosts, which means those 3 hosts become 93% utilized (70% utilized * 4 nodes / 3 nodes = 93.3% utilized). The more hosts you have in the cluster, the less utilized your cluster will be when putting a host in maintenance mode. For example: 70% utilized * 10 nodes / 9 nodes = 77.7% utilized after evacuation of a host.
The formula for this is:
% Utilization after evacuation = (% Utilization before evacuation * # nodes) / (# nodes – 1)