In Gittin 56b there is a conversation between Vespasian and R' Yochanan ben Zakkai. Vespasian offers to grant a wish of R' Yochanan's
אלא בעי מינאי מידי דאתן לך
R' Yochanan proceeds to ask for 3 things
אמר ליה תן לי יבנה וחכמיה ושושילתא דרבן גמליאל ואסוותא דמסיין ליה לרבי צדוק
How did R' Yochanan know he could ask for more than 1 thing and why did he stop at 3? I am aware that the gemarah discusses why he didn't ask to abandon the siege, but he could have asked for many other things.
Just to elaborate and add on to the above answers:
we see in the gemara (gittin 56b) that some chachamim said said about this "Who turns wise men backward and makes their knowledge foolish" as he could have asked to save the yidden however to explain why rybz didnt ask for this the gemara reasons that he knew if he would ask for that they wouldnt give it to him. according to this answer you could say he knew he would get these 3 things but nothing more. also he knew that the churban would happen (there were simanim happening for years befor this - the doors of the beis hamikdash opening by themselves, sanhedrin left the lishkas hagazis 4o years before, etc. he know his job was to guarantee the survival of torah and we can see today how succesfull he was!
Being there, he probably sensed what would be allowed both from the mood of the room and its contemporaneous propriety.
Further, he certainly sensed that it was Hashem who gave him this opportunity, especially when that was why he visited him in the first place. Therefore, he asked for what he thought appropriate, the amount being less important than what was actually requested.
