I am going to make some assumptions regarding your question. Let me know if any of my assumptions are incorrect, or if I can otherwise be of further assistance.
First, I'm going to assume that you are using Single Checbox fields, and not Mulitple Choice fields. Second, I'm going to assume that you are interested in adding up all of the selected checked boxes for one specific field, and not adding up all of the selected checkboxes for all fields together.
You can accomplish what you want using a SUMIF() expression. For example, if your Checkbox field is column B, then you can use a formula like: =sumif(@B, true)
I hope this helps,