As Many As, Not Even One (も)

Xも after a number always signals the speaker finds that number surprisingly high, 10時間も寝ました implies “that’s a lot of sleep.” Compare しか, which you already know pairs with a negative for “only,” も here works almost like its opposite, emphasizing a large or excessive amount.

誰も, どこも, 何も, いつも, all pair with a negative verb to sweep the whole category into the negative, “no one at all,” “nowhere at all,” “nothing at all.” Without the negative, these same words shift meaning entirely, いつも without a negative just means “always.”