Every language has, of course, its own set of words for numbers.  In English they are, one, two, three, etc. for the cardinal numbers and first, second, third, etc. for ordinal numbers.  The language has almost but not quite completely lost a third counting system sometimes found in other languages that counts the number of times something happens—once, twice, three times, four times, etc.  (Once and twice are preferred over one time and two times, but otherwise the ordinal number pattern is preferred.  There also remains a word thrice for three times but it is obsolete).

However, for prefixes, English does not use its own words, but, instead, words from other languages, mainly Latin.  For this reason it is useful to know how to count to ten in Latin: unus, duo, tres, quattor, quinque, sex, septem, octo, novem, decem.

You see these numbers in many English words: for example, triarchy,(rule by three);  duopololy (rule by two—especially an industry “ruled” by two companies); decimal, the Pentagon (a 5-sided building that is the headquarters of the U.S. military); decade; unit; unify; unicycle (also bicycle and tricycle).

Here are some other things that you can count by using the Latin number prefixes:

Babies:  two = twins, three = triplets, four = quadruplets, five = quintuplets, six = sextuplets, seven = septuplets, eight = octuplets.

Singers or musicians:  two = duet,  three = trio, four = quartet, five = quintet, six = sextet, seven = septet, eight = octet.

Multipliers:  times two = double, times three = triple, times four = quadruple, times five = quintuple,  times six = sextuple.

Two-dimensional shapes:  three = triangle, four = quadrilateral, five = pentagon, six = hexagon, seven = heptagon, eight = octagon, nine =  nonagon, ten = decagon.  (Three-dimensional shapes use the suffix -lateral – hence, trilateral, quadrilateral, etc.).  Sorry about six and seven, but English decided to use Greek prefixes for these.  There are types of quadrilateral that everyone knows—the square, the rectangle, the parallelogram and the trapezoid.  For those into physics, four dimensional shapes take the name of their three-dimensional equivalent but with the prefix super.  Hence, supercube.

Big numbers use a scheme in English based on the Latin numbers.   A thousand million is a billion, and each subsequent multiple of a thousand is, respectively, trillion, quadrillion, quintillion, sextillion, septillion, octillion.

You may have noticed a similarity between the months of the year and Latin numbers, except September is the ninth month, not the seventh, and so on with October, November and December (which is the twelfth month, not the tenth).  There are historical reason for this, but it can be marked down as just another attempt by the English language to be illogical.

The Latin word for a hundred is centum, so we see English words such as century, centimeter, and, of course, per cent (short for per cent or “per hundred.”)

The Latin word for a thousand is mille.  Hence millennium, millimeter.