Setting up replication for various mongodb micro instances
You have 5 servers serv1,serv2,serv3,serv4,serv5 and you want to set up 5 instanses inst1,inst2,inst3,inst4,inst5. Each instance should have at least 2 data nodes (PRIMARY and SECONDARY) and 1 ARBITER

serv1: inst1_PRI, inst5_SEC, inst4_ARB

serv2: inst2_PRI, inst1_SEC, inst5_ARB

serv3: inst3_PRI, inst2_SEC, inst1_ARB

serv4: inst4_PRI, inst3_SEC, inst2_ARB

serv5: inst5_PRI, inst4_SEC, inst3_ARB

This architecture provide fault tolerance of 1. If one server goes down all instances are continue to work. If you loose 2 servers one instance should be affected ...