ISIS recruitment is amazingly effective and with a potential pool of 100 million Sunnis recruits from ages 10-50 all over the world they basically have unlimited supply of cannon fodders.
Let's not get ahead of ourselves with those figures

ISIS has relatively large support among Sunni Muslims (some polls claim up to 80% like this
) , but most of those supporting them are basically keyboard warriors , unwilling to really take up arms and go in harms way .
On the ground , I doubt that ISIS has more then 100 000 fighters , mostly lightly armed . Combined offensive by world greatest powers (permanent members of UN Security Council : US, UK, France, Russia , China) would easily crush ISIS including most of their army . And their backers and supporters would be terrified for their lives to even attempt anything similar .
Problem is, ISIS is just one of many factions vying for influence in Middle East . And behind all of it are global struggles for power where ISIS represents just one of the pawns in great game . Therefore, as long we have Sunni-Shia conflict in ME , with support from global powers for both sides , ISIS and similar organizations could always be useful to achieve some strategic goal .