ses

Numerals never take endings. Ses stays ses whatever the noun beside it is doing, so it is mi vidis ses homojn, with the plural and accusative sitting on the noun alone. Adding them to the number is one of the commonest early slips.

Everything else is built with the ordinary endings on top: sesa is sixth, sesone is a sixth part, sesope is six of them together, and sesdek is sixty, written solid. Telling the time uses the ordinal, so la sesa is six o'clock with horo usually left out.