For each event and People profile that Mixpanel creates (i.e. Opportunity Created / Closed), Mixpanel has a limit of 250 properties.
If your Salesforce dataset object includes less than 250 default and custom properties, all properties will be imported. However, if your Salesforce dataset objects include more than 250 properties Mixpanel will just import the default properties.
In the scenario where one or more of the objects in your Salesforce dataset includes more than 250 properties, you can use a custom configuration to select the additional properties you’d like to include in the dataset import. Please refer to the article on specifying a custom configuration for more detail.