How do I add more people to my party size?

Currently I can only set my party to a maximum of 10 in the open table widget. However on the actual open table site you can set your party size to a max of 20 and there is even an option for larger party. I have seen these open table widgets being used on other sites and you are able to set your party size to a max of 20 or larger party size just like you can on the open table site. So how do I set up my widget to increase the party size to 20 or above ?

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