Hamas has indicated it is currently unable to identify and track down 40 Israeli hostages needed for the first phase of a ceasefire deal, according to an Israeli official and a source familiar with the discussions, raising fears that more hostages may be dead than are publicly known.
There are supposed to be 136 hostages in Gaza.
At one point both Israel and Hamas seemed to agree that there were approximately
200250 hostages.https://www.reuters.com/world/middle-east/what-do-we-now-about-hamas-hostages-2023-10-19/
Edit:
Wikipedia cites a slightly higher numberhttps://en.wikipedia.org/wiki/2023_Israeli–Palestinian_prisoner_exchange
Edit 2: my mistake, they corroborate their numbers (250)