The intermediate is a proctored exam with 50 multiple choice questions.
As preparation take the intermediate learning paths at https://academy.mendix.com/
Each learning path module ends with a knowledge check. These knowledge checks reflect the type, level and phrasing of questions you can expect during the exam.
Having 4-6 weeks of project experience a longside these learning paths will be sufficient preparation.
Do every academy course, take part in Mendix projects, preferably a scrum team, exchange knowledge with fellow Business consultants, follow live-builds on https://www.mendix.com/live/ and try to add sensible answers to forum-questions. Don’t yet specialise, but make sure you get experience in the entire Mendix spectrum, from interface to UI, from nanoflows to warning- and error-messages. After half a year or so, you will do fine on the intermediate exam.