"How many Finns can ride?"
Translation:Kuinka moni suomalainen osaa ratsastaa?
As an avid motorcyclist, the first thing that came to mind from the English sentence was riding motorcycles, which reminded me that it ought to be pointed out that "ratsastaa" is only used for riding animals. When referring to riding motorcycles, ATVs or bicycles, you should use the verb "ajaa" instead. Also, the verbs "polkupyöräillä", "pyöräillä" and "fillaroida" are additionally used for riding bicycles, and the verb "moottoripyöräillä" is additionally used for riding motorcycles. "Pyöräillä" is also used for riding motorcycles, but it seems far less common and is used mostly when it's clear from the context that it's specifically about motorcycling.
I hope we are offered the common plurals soon, these singular ones are not particularly useful day to day.
Kuinka monta (or montako) has more the meaning of "how many individuals". So how many people in a particular small group can ride horses, for example. They are countable.
Kuinka moni (or moniko) is more like a percentage or an approximate number of a larger group, in this case a whole population. So the answer could be X% of people or (rounded to) Y thousand people.
In practice, you may see Finnish speakers using these forms less strictly than this.