My experience from a couple of years back is 2 to 300 students after class and up to 30 or so in class during a quiz was possible. So a teacher sized Moodle then.
DO has one trick. You pay by the hour so if you want to run a simultaneous exam for all your students you can dial up CPU and RAM to a thousand dollar a month level. Within hours, you can take it back down so you pay maybe $10 for the metered six hours.
I have thought of setting up 50 virtual users doing a quiz in Moodle to see performance. Has anyone done that?