Nine are named after Presidents of the United States of America. But only eight presidents: J. Q. Adams, Jackson, Jefferson, Lincoln, Monroe, Pierce, Polk, and Washington. The ninth is Vernon County, which was named after Washington's home, Mount Vernon.
Eleven are named for Wisconsin politicians. No one you've probably heard of, unless you live in Wisconsin, and even then I wouldn't necessarily put money on it.
Four are named for other notable Wisconsinites, The man who founded what eventually became Milwaukee, for example, or Florence Julst, the first white woman to settle in that part of the state. There's another county named after an American Indian woman, though you wouldn't know it from her name: Marie Antoinette Chevalier. (She was married to a French fur trader, apparently.)
Five were named for politicians from out of state. My home county, for example, is named for Nathan Dane, a delegate representing Massachusetts in Continental Congress. Why? Because he drafted the Northwest Ordinance, which encouraged settlers to move into the Northwest Territory, out of which Wisconsin was eventually carved. But what about Reuben H. Walworth, a U.S. representative and judge from New York? Beats me. And then there's Ashland County, which like Vernon County was named after a politican's home, namely Ashland, Henry Clay's estate. But unlike Vernon County, there is no Clay County getting in the way of naming it after the man himself. Weird.
Four were named for American soldiers. Most fought in the American Revolution, but there's one War of 1812 veteran in the mix too.
Six were named after foreigners, mostly explorers but also a missionary and a French marquis who fought on our side in the American Revolution. Although come to think of it, the missionary was an explorer too.
Two were named after cities. Eau Claire County was named after the city of the same name, which became the county seat when the new county was carved out of an existing one. And Marathon County was named, oddly, after Marathon, Greece.
Twelve were named for natural features nearby. Including, bizarrely, Door County, which is named not for a door but for the strait that connects Green Bay and Lake Michigan, which is called Porte des Morts in French and "Death's Door" in English. Who knew? Similarly, Racine and Fond du Lac Counties take their names from French words for certain bodies of water; racine is French for "root," as in the Root River, which flow through the county, and fond du lac means "foot of the lake," specifically Lake Winnebago.
And the remaining 19 come from various tribal languages. It makes sense that a plurality of counties in Wisconsin would have tribal names, since the state itself has an Indian name of sorts; it's the English spelling of a misspelled French version of a Miami Indian name for what's now called the Wisconsin River. If Pere Marquette had had better handwriting, I might now live in Meskousing.