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 mean pressing 5400 keys in an 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.