"Remember that derivational morphemes can change the grammatical category/part of speech of a word. For example, adding -ful to beauty, changes the word from a noun to an adjective (beautiful). The form that results from the addition of a derivational mopheme (in this case, beautiful) is called a derived word/derivative."
So I think that the best way to find out how many derivational mophemes there are is to list all the morphemes you have learned about and see which ones you find in "impressionisms"
Here is a list of some morphemes:
* - nes
* -ly
* -tion
* -ment
* -al
* -able
* -ism
* un-
* pre-
* a-
* in-
