A bath tub has a capacity of around 110L.
An average 80kg person has a density of 1010kg/m3, therefore a volume of 0.08m3 or 80L. Let's say the person is only 50% under water when taking a bath.
That means around 70L of water are poured in the bath tub so that it's full to the rim when one person is sitting in it.
Compare with a shower. A typical shower would take around 10 minutes and shower heads have a water flow ranging from 6 to 16L per minute or 60L (water-saving shower heads) to 160L (normal shower heads) of water per 10m shower.
**In practice, water saving showers are equivalent to baths, whereas normal showers use up to 2.5 times the water with respect to baths.**