Birthmarks - marks present on the skin at birth - may be caused by:
1.) minor malformations of blood vessels;
2.) high concentrations of melanocytes in an area of skin;
4.) trauma whilst passing down the birth canal when being born.
Some are permanent; others are not, & fade/ disappear in early life.
"Classic" brown or black birthmarks may be due to genetic anomalies: some seem to be hereditary, appearing on the same areas of the bodies of different members of the same family; others occur randomly on individuals. "Mongolian marks" (usually bluish/ grey colour) occur frequently on babies of Asian ancestry. Brown birthmarks tend to occur on white people (5-10% frequency in most populations), although black people may have them too: they're most frequently located on the trunk, buttocks & legs; they occur more in women than men, & are most common on blondes/ fair skinned people, and especially redheads. Exactly why this is is uncertain, but it's most probably a genetic cause.
Such naevi are caused by an unusually high concentration of melanocytes in one area of skin; they may be regular or irregular in shape; large (ie >1.5 cm across) or small (commonly called "moles"); flat or raised; some may be hairy. Generally, such marks are harmless (cosmetic considerations aside), although multiple "cafe au lait" marks on one person may indicate certain neurological conditions.
The "Becker's Naevus" is a male "birthmark" - typically fairly large/ hairy, occurring on chest/ upper back - that emerges on some men (approx 0.2%) at or after puberty: seemingly stimulated by testosterone. Technically, because not present at birth, these are not birthmarks, but because they're almost certainly genetically determined (in a sense congenitally "programmed" to appear later) most regard them as such.
Other birthmarks are vascular - "port wine" marks are usually caused by abnormal dilation of blood vessels near the skin surface. If extensive, & on the head/ face, they may be associated with blood vessel abnormalities inside the head as well. Generally, however, such marks are harmless.
Many birthmarks are nothing more than the result of the traumas/ pressures experienced by the baby during birth: bruises/ contusions, minor extrusions of blood vessels etc (eg many so called "Strawberry marks"), and, in most cases, they fade and usually disappear in early life.
So, you "get" a birthmark either because of some minor developmental glitch whilst in the womb, or because of the traumas of birth itself.
There is no limit
Elliott Marks's birth name is Elliott Stephen Marks.
Alfred Marks's birth name is Alfred Edward Marks.
Arthur Marks's birth name is Arthur Ronald Marks.
Debbie Marks's birth name is Debra F. Marks.
Eugene Marks's birth name is Eugene Robert Marks.
Garnett Marks's birth name is Garnett Anderson Marks.
Griffin Marks's birth name is Griffin Daniel Marks.
Jimmy Marks's birth name is James Stanko Marks.
Louis Marks's birth name is Louis Frank Marks.
Torey Marks's birth name is Victoria Hope Marks.
Carolyn Marks Blackwood's birth name is Carolyn G. Marks.