The noun mileage can be countable or uncountable. In more general, commonly used, contexts, the plural form will also be mileage. However, in more specific contexts, the plural form can also be mileages e.g. in reference to various types of mileages or a collection of mileages.

Is 20 miles plural or singular?

Miles is plural. (But we say 0.5 miles and not 1\2 mile, we say 1\2 a mile. Even though they are mathematically the same quantity.

Is it 0.1 mile or miles?

Point one mile is one tenth, or 0.1 mile. British English speaker here, with a different answer. Yes, you use the plural when it is a decimal. A mile is one mile.