Assuming no faithless electors: eleven. These are the states with the most electoral votes:
California (55 votes)
Texas (38 votes)
Florida (29 votes)
New York (29 votes)
Illinois (20 votes)
Pennsylvania (20 votes)
Ohio (18 votes)
Georgia (16 votes)
Michigan (16 votes)
North Carolina (15 votes)
New Jersey (14 votes)
If somebody managed to win those 11 states and no others, that person would have a majority in the electoral college and win the presidency.
Realistically, though, this won’t happen. California, New York, Illinois and New Jersey are heavily Democratic and won’t vote Republican unless there’s a huge landslide that would mean a lot of states that are normally reliably Democratic would vote Republican. Texas and Georgia are reliably Republican