You can create multiple custom fields in calendar module by following the given steps:

• Go to Settings > Module Manager.
• Select the tab of Standard Modules.
• Find the Calendar module and click the icon of Calendar Settings.
• Click on calendar custom fields.
• Select the radio button of either Event or Task for which you want to create custom field.
• Click New Custom Field button.
• Select the Field Type and mention its Label and any other required field.
• Click Save.

You can verify the presence of new field in the create activity window.