This depends on a variety of factors: the height of the load, its distribution on the tray, size and shape of culture vessels baffled or not baffled, the shaking throw, the model of Multitron, the position in the stack and the type of base (high or low) for stacked systems. Graphs are provided in the operating manuals as a guide and these are based on standard Erlenmeyer flasks filled typically at 20-25%. 
Special adaptations can be made for almost any type of load. Please contact your INFORS HT representative with details of your specific application.