Too many Enders Dragons!

I have too many dragons in my survival world seeds. When I prepare all to finally go to the end and kill my ender dragon, SURPRISE! ¡51 enders Dragons spawn in the island!, off course Lag the game and 3 seconds after that, I die  for severals hits and 'fireballs' at the same time. I enter again for check if only spawn those 51 dragos one time... But not, still there... Plis fix that, this situation are really annoying. I Adjunt a screenshot from a copy of my world, I use there the command '/kill @e' to count all the dragons, look that, is insane. Thnx.