As per the new plot system rules (you should give those a read if you haven't already), D100 is the default, and a table is provided as well. But you can use other dice as needed (for like rolling for number of opposing NPCs). Here is an of example of a dice roll.
To roll multiples of the same dice. You can replace the first number for how many dices you want to roll. For example, the following will roll three d100 dices in a row.
To roll multiples of different numbered dice, you will need to separate them with a semi-colon and a space like so for example.
[dice]d6; d20; d100[/dice]
Be sure and remember that you can only use one line of the dice code per post. Either roll multiple dice in the same code (as shown above) or post them in separate posts. If you don't all rolls will get the same result. This is because the dice code when used is saved to a post ID.