Platform event limits apply to a 24-hour cycle. Before delivering new events, such as a change in order status, to subscribed clients, Salesforce checks your allocation and how many events have been used in your Salesforce org in the last 24 hours. If the number of events used is lower than the 24-hour allocation, Salesforce delivers events to subscribers. If you did hit your allocation for events in the previous 24 hours, Salesforce doesn’t deliver new events. But, they’re queued within a brief time window until event allocation is available.
Reference Trailhead article – https://trailhead.salesforce.com/de/content/learn/modules/app-development-without-limits/app-development-without-limits-rate
If the user has multiple browser tabs using empApi, the streaming connection is shared and is counted as one client for that user. A client that exceeds the concurrent client allocation receives an error and can’t subscribe. When one of the clients disconnects and a connection is available, the new client can subscribe
Reference Article – https://developer.salesforce.com/docs/atlas.en-us.platform_events.meta/platform_events/platform_event_limits.htm
For other limitations in Platform Event – https://developer.salesforce.com/docs/atlas.en-us.212.0.platform_events.meta/platform_events/platform_event_limits.htm
Sample Platform Event Example – https://infallibletemp.com/2017/06/platform-events-in-salesforce.html