Some keyboards limit the number of keys pressed and registered at the same time. If you are unable to play multiple keys simultaneously, it is most likely because of the limitation of your keyboard. Alternatively, you may use an iPad/tablet which enables you to touch multiple keys at the same time. On average most of the good keyboards will allow 4 keys to be pressed simultaneously. Although, if the keys are in the same row, it's often possible to press more than 6 at the same time. If you have 4 or 5 keys in brackets that need to be pressed simultaneously but your keyboard doesn't allow it, one workaround is to press those keys very fast one after another and your input will still be registered as correct by Virtual Piano.