Creating A For Loop To Detect How Many Selects Are On Form

On my form i have multiple select which all have an id value total1, total2,
total3 etc so i am trying to detect how many there are and then use this to
caculate a total.

Is there a javascript reference to basically go to a form and produce a loop
which will show me how many select drop down boxes there on a form.

Or would i have t use something like

for (var i=0; i < frm.elements.length; ++i) {
form.elements.length
form_field = frm.elements[i]

and then have a nested if to detect if it is select value or has an matching
value ..