How many Earth hours is in one Mars day?

The simple answer is "just over 24 hours".

More details:

A day on Mars is 24 hours, 39 minutes and 35 seconds

That's the "solar day". The rotation period or "sidereal day" is

a couple of minutes less.

For comparison, on Earth the solar day is 24 hours and the sidereal day is about 23 hours and 56 minutes.