I listened to an interview with Энджел Coulby and she сказал(-а) that Merlin will only have 5 season. Has that been confirmed??

No that is not confirmed. The creators (said in an interview) began the project with a five год plan which is not uncommon with tv shows. So the Показать could definitely run beyond 5 years. Now that the Показать has been commissioned for the 5 years the can start planning for the possibility of 6.. 7.. 8 etc.