Hello Salar, 

I am sorry to hear that you are seeing issues creating more than 20 domains. How close are you to your user limit? Your SmarterMail license is based off of the number of users you have and by default we create a new user with every domain. 

You can view this information on your license page. 

Are you getting any errors when you try to add the domain? Check the error log as well as any pops up that may be happening. 

Thank you