Merge and Custom Fields for Mailchimp - Updated Dec 2019

 Mailchimp has limits to the number of fields, you can use your own fields as well just make sure they are not required.

• Audiences are limited to a maximum of 30 data fields for each contact. 
• If you want more than 30 fields of information about your contacts, use groups to help consolidate audience fields, or use tags instead.
• The data field limit is higher for Mailchimp Pro accounts, which can use up to 80 fields in each audience. 
• If Pro is disabled, the extra 50 audience fields won't be visible.