There are 3141 active counties, 947 obsolete, and 208 renamed in the United States all dating from the mid-1600s to the year 2000. This is the data found in AniMap.
New counties were unorganized, assigned to an organized county for judicial and government purposes. This means until the county is fully organized, records are found here.
It is known locations did not move throughout history, however, county boundaries did often leaving you searching in the wrong county depending on the date of the event.