Are these machines in 1 VCenter?

My guess is that when 1 server crashes it will try to migrate them to the other server but because the source server is down it will fail. It will not recreate the machines automatically.

AFAIK you only have 2 options. Either make sure to use shared storage (either a SAN or VMWare's VSAN which is quite expensive licens wise) or create 100 machines on both server and make sure to not use them all.. This way you can still have 100 machines left if one of the ESX hosts fails.

Maybe someone else has a better idea??