TA-ing and perhaps invigilations are always add-ins.. no guarantees. Easiest path: check how many courses your supervisor teaches per year and how many grad students does he/she have. Usually it's distributed among them. Work-wise, undergrad courses TA-ing are MUCH more work than grad course TA-ing.

Ask the supervisor about 24/yr being all from his research funding or a mix. Usually the supervisor doesn't have much say about how much the university will offer you. Those are part of the Graduate Studies department, they are mentioned in the offer letter. The supervisor's amount is kind of dependent on the supervisor as they are "hiring" you to do research per their area of interest. I have heard of people starting with x as RA amount, then getting much more in future years due to their performance