Till CRM 2015 update 1, we had upper limit of 200 Workflows, 300 Dialogues and 300 Custom Entities. From CRM 2015 Update 1 onwards, the limit of workfows, dialogues and custom entities have been removed.
In Dynamics 365 Online we can still see information related to custom entities which shows upper limit as 300. However this is more of indicative and there is no upper limit now. Please refer the below link for more details
Go to Settings –> Administration –> Resources in Use
Hope it helps..