I know nothing about this keyboard, but I'd assume it just has one solenoid because the expense and space of 100+ solenoids is impractical if all you're using them for is simulating the vibration/sound of a typewriter.
>>Gh0stR+(OP)
I wish I could delete my comment to hide my stupidity. For some reason I was thinking about springs despite reading and typing solenoid. You are of course 100% correct and unfortunately it's too late for me to hide my shame.