Slot was speaking ahead of their Carabao Cup semifinal against Tottenham on Thursday night. Liverpool host the second-leg trailing 1-0.
Slot admits he's baffled by the limits on squad numbers: "That is one of the only things that I don't understand here in England. Everywhere around the world and in Europe as well when we play Champions League, we can take 23 players to the game. We are here in a country where we play the most games - especially the ones like us, Arsenal, Aston Villa, Chelsea - all the ones that are playing in Europe.
"We play so, so many games. If you are able to play all of these games, you need a big squad, that seems to be fair to me. One of the things that is so important for a manager - to pick the first 11 is difficult - but to tell someone they aren't even in the squad makes it even harder. I don't understand why we don't go to 23 in this country because this country plays the most games out of anywhere in the world and we only have 20 players we can take to the game. It's not for that game because nine substitute is more than enough but it would keep the dressing room going.
"You need more than 20 players in this country because we play so many games. Why can't we bring 23 players to the game instead of 20? That will be the most difficult decision I have to make tomorrow - to leave someone out who works so hard on a daily basis and wants to be involved - that is the hardest decision for me to make."
Slot was also asked about who will be in goal tomorrow night - Alisson Becker and Caoimhin Kelleher.
He added, "The two goalkeepers don't know yet. When I leave this room, I am going to go to the media next door and then afterwards I will inform the goalkeepers which decision I have made."