The micro:bit has two buttons, as well as the accelerometer. You can use different inputs to roll different types of dice.
For example,
What types of dice would be useful in a game you enjoy?
Additional if statements need to be inside your while True loop. They must be indented to the same level as your current if statement.
Take a look at this structure:
while True:
if accelerometer.was_gesture("shake"):
roll = randint(1, 6)
display.show(roll)
sleep(5000)
display.clear()
if button_a.was_pressed():
display.show("A")
if button_b.was_pressed():
display.show("B")