how many grams of the sample are given to the students? If they run out, can they ask for more without penalty?

It will be up to the Event Supervisors. The Event Supervisors may notify the students at the beginning of the competition if refills will be allowed or not, students may ask the Event Supervisor.  Since materials vary widely in density, there is no minimum or maximum number of grams Event Supervisors are to provide to the students.