Can I dispute everything in one letter or should I put each dispute in a separate letter?

As far as I know, there is no reason you should not dispute all negative entries in one letter. For example, you have 3 outdated entries and 2 incorrect entries. Write one letter disputing all 5 entries. If you’d prefer to write 2 letters, one for the outdated entries and one for the incorrect, you can do that, too.