I searched and read google documentation about this subject, but I don't see good text about FCM. This link have pricing about all module exception cloud messaging,
My question :
How many device can register to fire base in free plan?
How many can send message per day in free plan?
There are no documented limits on the number of devices nor on the number of messages that can be registered in Firebase Cloud Messaging. This no-limits applies equally to both projects on paid and on free plans.