Once you attack them, you will be able to see the number of captains in their clan on the battle page.
I wish that we could look at the comparison other than immediately after attacking them; like from the "rivals" screen or the battle screen or even from their profile .. *if* we have attacked them in ... say.. the last hour. I just have trouble where I do a click accidentally that removes the "show details" and then I've lost it forever. Also... I really rely on the new inventory format that shows what I'm using on defense. Thanks. I wish Viking had it.