There is no limit to the number of Events you can send to Mixpanel. There are, however, UI limits on the number of different Event names that can be displayed in Event dropdowns in your reports. Event dropdowns hold a maximum of 1,500 different Event names.
Which Events will show up if I have more than 1,500?
Events that have not been sent to Mixpanel in 30 days will be automatically hidden from the dropdown (read more on hidden Events and Properties and how to unhide them).
Other than that, your most frequently fired Events will be displayed in the dropdown if you have more than 1,500 and all of them have been fired in the last 30 days.
It is best practice to keep your list of Events short and use Properties to give more context and detail on those Events.