Docchi, Donogurai and Ikutsu
Here are explanations for these three words.
docchi: meaning “which one” only when you have two alternatives like meat or fish. It often refers to directions like this way or that way. You need to use dore when you have more than two options.
niku to sakana, docchi ga ii desu ka?: Which one do you prefer meat or fish?
ichigo to choco to maccha, dore ga ii desu ka?: Which one do you want strawberry, chocolate or green tea?
donogurai / donokurai : refers to time, distance or degree.
Tokyo kara Osaka made donogurai (or nanjikan) kakarimasu ka?: How long (or how many hours) does it take to travel from Tokyo to Osaka?
Tokyo kara Osaka made donogurai (or nankilo) desu ka?: How long (or how many kilometers) is it from to Tokyo to Osaka?
kono shiken wa donogurai muzukashii desu ka?: How difficult will this test be?
ikutsu: A general counter for objects, so you can use “ikutsu” for anything you want to know the number of. As you probably know, there are many counter words for certain types of obejects. Japanese people usually choose the appropriate counter word over “ikutsu” according to the object in question.
ikutsu (or nanko) ringo o kaimashita ka?: How many apples did you buy?
koko ni ikutsu (or nanbon) pen ga arimasu ka?: How many pens are here?