If by some amazing chance all the tiles two and three hexes out from an enemy city were flatland, you could set up 42 Keshiks in all the tiles three and four hexes away and do the Keshik dance with every single one. How many HP does a city have, anyhow? Well, I guess you would actually want 41 Keshiks and then a horseman four away to take the city when it's zero.
I also couldn't believe how crazy Keshik were when I first played them. Dancing in to attack and dancing back out was wild. Although sometimes once I get march, I stay in range and take the counterattacks for the XP as long as the city isn't doing more than two damage to me.
EDIT: OK, I found the city HP is 25, so I guess you only need 24 or 25 Keshiks, depending on if you take the city when it reaches zero HP or when I goes under zero HP.