But it is true that there are 75 forms of the word "eighty"*. Here's how: there are 14 cases in Estonian. Each case has a singular and plural form, making 28 total. The numbers, in addition, have cardinal (one, two, three) and ordinal (first, second, third) forms, which makes two sets of 28. That makes 56, but in some cases there are more than one possible form. For the word "eighty" (or for any similar number word), many plural cases have two forms, and the total is 75 forms.
On the bright side, there are fairly consistent rules to how cases are formed, so for most words you can form the all the other cases by learning only three forms (nominative, genitive, and partitive).
*Why did I choose the number eighty? Because it's the one I encountered recently that was giving me trouble. I was trying to figure out the adessive plural of the ordinal number; in other words, I wanted to say "in the eighties". No, English doesn't use the ordinal number for this, but Estonian does: it means "in the eightieth (years)."
By the way, the correct form for "in the eighties" is kaheksakümnendatel.
Põhiarvud - cardinal Ainsus - singular forms nimetav (nominative) omastav (genitive) osastav (partitive) 1 osastav (partitive) 2 sisseütlev (illative) seesütlev (inessive) seestütlev (elative) alaleütlev (allative) alalütlev (adessive) alaltütlev (ablative) saav (translative) rajav (terminative) olev (essive) ilmaütlev (abessive) kaasaütlev (comitative) Põhiarvud - cardinal Mitmus - plural forms nimetav (nominative) omastav (genitive) osastav (partitive) sisseütlev (illative) 1 sisseütlev (illative) 2 seesütlev (inessive) 1 seesütlev (inessive) 2 seestütlev (elative) 1 seestütlev (elative) 2 alaleütlev (allative) 1 alaleütlev (allative) 2 alalütlev (adessive) 1 alalütlev (adessive) 2 alaltütlev (ablative) 1 alaltütlev (ablative) 2 saav (translative) 1 saav (translative) 2 rajav (terminative) 1 rajav (terminative) 2 olev (essive) 1 olev (essive) 2 ilmaütlev (abessive) kaasaütlev (comitative) Järgarvud - ordinal Ainsus - singular forms nimetav (nominative) omastav (genitive) osastav (partitive) sisseütlev (illative) seesütlev (inessive) seestütlev (elative) alaleütlev (allative) alalütlev (adessive) alaltütlev (ablative) saav (translative) rajav (terminative) olev (essive) ilmaütlev (abessive) kaasaütlev (comitative) Järgarvud - ordinal Mitmus - plural forms nimetav (nominative) omastav (genitive) osastav (partitive) sisseütlev (illative) 1 sisseütlev (illative) 2 seesütlev (inessive) 1 seesütlev (inessive) 2 seestütlev (elative) 1 seestütlev (elative) 2 alaleütlev (allative) 1 alaleütlev (allative) 2 alalütlev (adessive) 1 alalütlev (adessive) 2 alaltütlev (ablative) 1 alaltütlev (ablative) 2 saav (translative) 1 saav (translative) 2 rajav (terminative) 1 rajav (terminative) 2 olev (essive) 1 olev (essive) 2 ilmaütlev (abessive) kaasaütlev (comitative) | kaheksakümmend kaheksakümne kaheksatkümmend kaheksatkümmet kaheksakümnesse kaheksakümnes kaheksakümnest kaheksakümnele kaheksakümnel kaheksakümnelt kaheksakümneks kaheksakümneni kaheksakümnena kaheksakümneta kaheksakümnega kaheksakümned kaheksakümnete kaheksakümneid kaheksakümneisse kaheksakümnetesse kaheksakümneis kaheksakümnetes kaheksakümneist kaheksakümnetest kaheksakümneile kaheksakümnetele kaheksakümneil kaheksakümnetel kaheksakümneilt kaheksakümnetelt kaheksakümneiks kaheksakümneteks kaheksakümneini kaheksakümneteni kaheksakümneina kaheksakümnetena kaheksakümneteta kaheksakümnetega kaheksakümnes kaheksakümnenda kaheksakümnendat kaheksakümnendasse kaheksakümnendas kaheksakümnendast kaheksakümnendale kaheksakümnendal kaheksakümnendalt kaheksakümnendaks kaheksakümnendani kaheksakümnendana kaheksakümnendata kaheksakümnendaga kaheksakümnendad kaheksakümnendate kaheksakümnendaid kaheksakümnendaisse kaheksakümnendatesse kaheksakümnendais kaheksakümnendates kaheksakümnendaist kaheksakümnendatest kaheksakümnendaile kaheksakümnendatele kaheksakümnendail kaheksakümnendatel kaheksakümnendailt kaheksakümnendatelt kaheksakümnendaiks kaheksakümnendateks kaheksakümnendaini kaheksakümnendateni kaheksakümnendaina kaheksakümnendatena kaheksakümnendateta kaheksakümnendatega |