How many INPUT_PULLUPs can I use on a UNO

I have looked for an answer (honestly!).
This is a most useful feature and I have at least 3 switches I would like to use it on because it saves external circuitry.
I can't work out if there is only one internal pull up resistor and if this can be 'shared'.
So far two INPUT_PULLUPs work fine but is there a limit ?
Obviously I'm pulling the pin to GND when the switch operates.

Each digital pin has it's own internal pullup resister. No worries.

Use as many as you like. There is no restriction and that includes the analogue pins too

Thanks for the quick and helpful response.
They are particularly useful for opto-isolators
Steve