After making a statement, the question “Need I say more?” is often said humorously or rhetorically. It means that everyone listening should be able to guess the result of what you have just said or means that you really don’t need someone to answer the question. It should be obvious.
Notice the rare and unusual form:
‘Need’ is a modal auxiliary verb. Therefore, there is no ‘DO’ to make the question and there is no ‘TO’ before the infinitive ‘say’.
Here are another two overlapping C2 grammar points in the category of MODALITY that could be joined into one point. The first is general enough to contain the specific use of the second point.
need – question form rhetorically in discussion, typically in formal contexts.
An iWeb search for:
need _P _V * ?
1 NEED I SAY MORE ? 1972
2 NEED WE SAY MORE ? 984
3 NEED I GO ON ? 273
4 NEED WE GO ON ? 44
5 NEED I SAY ANYMORE ? 19
6 NEED I SAY IT ? 12
7 NEED WE SAY ANYMORE ? 10