Salesforce limits the number of time triggers an organization can execute per hour as follows:
Enterprise Edition =500 Developer Edition =50 Professional Edition=250
Unlimited Edition =1,000
If an organization exceeds these limits, Salesforce defers the execution of the additional time triggers to the next hour. For example, if an Unlimited Edition organization has 1,200 time triggers scheduled to execute between 4:00 PM and 5:00 PM, Salesforce processes 1,000 time triggers between 4:00 PM and 5:00 PM and the remaining 200 time triggers between 5:00 PM and 6:00 PM.
Please check the below link for more details:
Hourly limit exceeded for processing workflow time triggers