Here is an easy one
This sentence has ______ letters.
Fill in the blanks in with a number in words.
There are two solutions with the implied constraint of the words being numbers:
"thirty-one" and "thirty-three".
However, there are several more possibilities:
"more than two" (as @Zandar said)
"several"
"not exactly fifty"
""
"spaces and"
"an underscore in place of"
"a period after forty-two"
"144 characters, 11 of which are digits, 26 of which are spaces, 4 of which are punctuation, and finally 105 of which are"
(Check that last one out, it works.)