If the field contains 'yes' or is blank, you can use a rollup with COUNTA(values) for the aggregation function.

If the field contains 'yes', 'no', 'maybe', and so on (that is, if you’re looking for yesses among a number of possible results), you’ll need to define a formula field with the formula

IF({Query Field]='Yes',1,0)

and then roll up the values of that field using SUM(values) as an aggregation function.