I have ten tickets for the soccer game on Sunday.
My breakfast is usually just one slice of toast.
I’d like to buy some Tofugu stickers. How much would one be?
This is a counter that can be used with various "flat" things, which is why it is referring to a flat object. Other things it can refer to include clothing items, sheets, paper, stamps, dishes, and more. Notice how they're all kind of flat shaped?
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.