I want to use Firebase notification for my android application and what i want to know is that is there any limitation for number of topics ? or for the number of users that can subscribe a topic ? For example can i have 10000 topics with 1 million users for each of them ?
There is no limitation on the number of topics or subscriptions. There was a limitation of 1 million subscriptions for the first year after topics was initially launched but that restriction was removed at this year's (2016) Google I/O. FCM now supports unlimited topics and subscribers per app. See the docs for confirmation.