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[GoT] [SPOILER] Why does Jamie tell Cersei that they can’t beat the Dothraki?

Just a random thought that came up today. I know that Dany and her dragons were a big reason she won that battle, but it sounds like Jamie is stating that it’s the Dothraki themselves that couldn’t be beaten. What are his reasons for thinking this? Couldn’t a horse-focused army be held at bay with spears and pikes? If I’m over-simplifying the situation, please let me know.
gpt
Because of sheer number. By the start of Season 7, Jaime commented that Cersei was at best the Queen of three Kingdoms. The North, Tyrells, Martells were not on their side, considering the events of the previous seasons. The Freys were wiped out by Arya so that left just the Lannister, Baratheons and maybe Harrenhal which wouldn't even amount to how much the Dothraki had and that was assuming just the Dothraki.

Remember that even King Robert commented that it would be stupid to meet the Dothraki in an open field.