1. An image forming apparatus that executes an image forming process based on print data including an execution command of transparency processes to be executed, comprising: a first storage unit to be used when the transparency process is executed; and a control unit for controlling the first number of transparency processes to be executed when a storage capacity of the first storage unit is insufficient to execute the transparency processes to be executed, and causing a display unit to display a preview image to which the second number of executable transparency processes different from the first number has been executed to determine an image being an object for print output based on an instruction by an operator relating to which transparency process is executed.