A keystroke means to type a character into a keyboard (not to press all the keys simultaneously). Every time you hit a key, you perform a keystroke. Therefore, 5400 keystrokes per hour means to press 5400 keys in one hour, or 90 keys in a minute (5400 ÷ 60 minutes).
Kabar keystrokes per hour (KSPH) or keystrokes per minute (KSPM) are sometimes used to measure typing speed, instead of words per minute (WPM). This is because typing the word “hi” 50 times will never be the same as typing a big word like “Nebuchadnezzar” 50 times.