What words refer to a group of a particular number?

single, duo, duet, trio, quartet, quintet, sextet, twins, triplets, quadruplets, quintuplets, sextuplets, twosome, threesome, foursome, fortnight, decade, century, millennium, dozen, score, gross,

What words refer to a group of two?

pair, couple, couplet, brace, dichotomy, deuce, dual, dyad, twain, yoke,

What words indicate that something is done in groups of a certain number?

singly, one by one, two by two, three by three, by ones, by twos, by threes, in pairs, in threes, one at a time, two at a time, three at a time,