I want to have a checkbox if based on the values from a combination of other checkboxes, ex; if 2 or more of 7 checkboxes are filled I want checkbox 8 to autofill. if only 1 of other 7 checkboxes are filled checkbox 8 does not autofill and is left b

I would do it using a (hidden) text field with the following calculation script (let's say the fields are named Check1 to Check8):

var ticked = 0;
for (var i=1; i<=7; i++) {
    if (this.getField("Check"+i).valueAsString!="Off") ticked++;
}
this.getField("Check8").tickThisBox(0, (ticked>=2));

The main benefit of doing it like that is that it will also work when the form is cleared, while the MouseUp approach won't.

Also, you have a single location with all the code necessary and you don't need to apply the same function to multiple fields.

So you need to count the checked boxes. The best way to do this is to use a group naming convention for the first 7 checks, like this: "MyGroup.check1", "MyGroup.check2",  etc.

Then this code will return true/false based on your criteria

   var nCnt = 0;

   this.getField("MyGroup").getArray().foreach(function(a){if(a.value == "Yes") nCnt++;});

    this.getField("Check8").value = (nCnt>1)?"Yes":"Off";

Place this code in the MouseUp event of each of the 7 checkboxes.