Suppose there is a finite maximal length L for questions to be readable and answerable. Denote, in addition, the number of symbols in all known human languages (including punctuation, spaces, mathematical symbols, etc.) by M.
Then the number of possible questions is bounded by (i.e. <) M^{L}, that is finite (but quite large). This is a strict upper bound since there are semantic and grammatical constraints in constructing sentences that are not taken into account here. Correspondingly, the number of possible answers of maximal length L' would be bounded by M^{L'}.
The only way to have an infinite number of questions and answers is to allow for questions and answers of infinite length (perhaps recursively as in another answer is suggested). However, any such question of infinite length would be impossible to read or write for any finite number of humans, and since with the utmost probability humanity will be present only for a finite timespan (bounded by the sun's lifespan as an active star), it will then be impossible to read or write even by mankind as a whole.
As a side note, this type of question reminds me of J. L. Borges story "La Bilblioteca de Babel", where he imagines a library containing all possible books (with a standardized format, and written only with latin characters). The main character of the novel at some point argues that the library is infinite, however it is not, by the same reasoning as above (nonetheless, for the purpose of the story, it is much more poignant that the library is actually infinite - and it could be, provided there are (infinitely many) duplicate books).