/summon MOB ~ ~1 ~ {Riding:{id:"MOB"}} =2 mobs
/summon MOB ~ ~1 ~ {Riding:{id:"MOB",Riding:{id:"MOB"}}} =3 mobs
/summon MOB ~ ~1 ~ {Riding:{id:"MOB",Riding:{id:"MOB",Riding:{id:"MOB"}}}} =4 mobs
/summon MOB ~ ~1 ~ {Riding:{id:"MOB",Riding:{id:"MOB",Riding:{id:"MOB",Riding:{id:"MOB"}}}}} =5 MOBS
My creations
/summon Ghast ~ ~1 ~ {Riding:{id:"Giant"}} =giant with a ghast head that shoots fireballs
/summon Chicken ~ ~1 ~ {Riding:{ActiveEffects:[{Id:1,Amplifier:1,Duration:999999,ShowParticles:0b}],id:"Bat"}} =flying chicken
/summon EntityHorse ~ ~1 ~ {Riding:{id:"Cow",Riding:{id:"Pig",Riding:{id:"Ozelot",Riding:{id:"Chicken",Riding:{id:"Bat"}}}}}} =Flying Animal Party
/summon Minecart ~ ~1 ~ {Riding:{id:"EnderDragon"}} =rideable ender dragon (you have to click the minecart)
/summon Minecart ~ ~1 ~ {Riding:{id:"Ghast"}} =Rideable Ghast
/summon Creeper ~ ~1 ~ {ActiveEffects:[{Id:1,Amplifier:1,Duration:999999,ShowParticles:0b}],Riding:{id:"Bat"}} =Exploding Suprise (invisible creeper riding bat)