I need a formula to get the count of all possible dropdown options in a text-multiple choice field.

Well, one solution is to copy and paste those 65 options into a new table with 65 records. Set the key field to be the text value of those choices. Then make a relationship back to your app based on the current multiple choice field as the reference field. Basically change it from a multiple-choice field to a table relationship. Then you can easily have formulas and some summary fields which total the number of records in the table.