USA 2023 Rulebook, Rule 6C.1.B
(Seniors) The pitcher may take a position from the front edge of the pitcher’s plate
to ten feet behind the pitcher’s plate within the 24 inch width of the pitcher’s plate
with both feet firmly on the ground.
So it's up to 10 feet behind the front edge of the pitcher's plate and within that 24 inch width.
USA Rulebook is available as a PDF on the USA Softball site for no charge, FWIW. This is new compared to the past where you had to either buy the app, or get a printed copy from your association.