This depends on the learning management system being used. The maximum number of answer choices is 20 for the Blackboard Learn. The number is 26 for D2L Brightspace, Moodle, Instructure Canvas, and the IMS QTI personality.
Moodle, Blackboard Learn and newer versions of other learning management systems support more than 26 answer choices where additional answers can be added after the question is published to the course. However, for questions with more than 26 answers in the LMS, Respondus 4 will only retrieve the first 26 answers in the question.