Ordinal numbers are ordered numbers. In English ordinals are numbers like "first", "second", "third". Here are the basic ordinal numbers in Latin with the Roman numeral corresponding to value. The ordinal numbers are declined like first and second declension adjectives. There are some oddities to note:
Variable presence of "n" before "s"
for "21st" in the feminine you might see una et vicesima or the contracted form unetvicesima.
For other compounds, there is variety. You may see the larger number before the smaller with no conjoining "et" or you might see the smaller before with larger separated by the conjunction "et". Thus, you may see either vicesimus quartus (no et) or quartus et vicesimus (with et). For 28th, the Latin ordinal number is based on the idea of taking 2 from 30 or duodetricensimus, just as the duo de '2 from' precedes 20th in the ordinal number for 18th: duodevicesimus.
I. primus (-a, -um)
II. secundus, alter
III. tertius
IV. quartus
V. quintus
VI. sextus
VII. septimus
VIII. octavus
IX. nonus
X. decimus
XI. undecimus
XII. duodecimus
XIII. tertius decimus or decimus et tertius
XIV. quartus decimus or decimus et quartus
XV. quintus decimus or decimus et quintus
XVI. sextus decimus or decimus et sextus
XVII. septimus decimus or decimus et septimus
XVIII. duodevice(n)simus, also octavus decimus
XIX. undevice(n)simus, also nonus decimus
XX. vice(n)simus
XXI. unus et vice(n)simus, also vicesimus primus
XXII. alter et vice(n)simus or vicesimus secundus
XXX. trice(n)simus or trigesimus
XL. quadrage(n)simus
L. quinquage(n)simus
LX. sexage(n)simus
LXX. septuage(n)simus
LXXX. octoge(n)simus
XC. nonage(n)simus
C. cente(n)simus
CC. ducente(n)simus
CCC. trecentensimus
CCCC. quadringentensimus
D. quingentensimus
DC. sescentensimus
DCC. septingentensimus
DCCC. octingentensimus
DCCCC. nongentensimus
M. millensimus
MM. bis millensimus