There is no definitive answer to this question as it depends on how you measure "greatest number of miles." Some sources might count the total length of Interstate roadways within a state, while others might only count the length of Interstate highways that are open to traffic.
In terms of total length, Texas has the longest Interstate system, with over 1,500 miles of road. Other states with long Interstate systems include California, Florida, and New York.
However, if you only count the length of Interstate highways that are open to traffic, the state with the greatest number of miles would be Missouri, with over 900 miles of Interstate highways. Other states with high numbers of open Interstate miles include Kansas, Oklahoma, and North Carolina.