Why is a persons name classed as a countable noun?

Consider this.

In my class there is one Clive, one Betty, three Mikes and six Toms. That's a total of eleven people.

Or consider this excerpt from Wikipedia

The Three Marys (or Maries) refer to the three biblical Marys that came to the sepulchre of Jesus in the Gospels and are companions of Mary, the mother of Jesus. All four gospels include a mention of the incident, but only Mark (16:1) identifies all three. In the verse, the three are:
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Goliath is known for his strength.

Shalini's hair is very long

In tha above 2 sentence which is countable and uncountable noun

Is Mr Mohan a countable.
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