What is Borderline Personality? A doctor once mentioned I might be- does that mean I am not really one person but a bunch?

Borderline Personality Disorder as defined by the DSM 1V (Diagnostic Manual used by psychiatrists and psychologists for diagnostic purposes) is
characterized by a pattern of instability in interpersonal relationships,
self-image and affects (emotions) and acts of potentially self-damaging
impulsivity (spending, substance abuse, sex, binge eating, driving).

The concept of "not really one person but a bunch" falls under the diagnosis
of Dissociative Identity Disorder(formerly Multiple Personality Disorder)
and is characterized (defined in the DSM 1V) by the presence of two or more distinct identities or personality states that take control of an
individual's behavior in addition to an inability to remember pertinent
personal information.

You can see that the descriptions above reflect two quite distinct
conditions and it would be useful, if you have not already, to speak with
your physician to further clarify this diagnosis and perhaps to conduct some
of your own exploration by reading. A very useful resource on Borderline
Personality Disorder is J.J. Kreisman, M.D. and Hal Straus's book entitled
" I Hate You-don't leave me".

Working with a psychiatrist or psychologist can be immensely helpful when coping, both intellectually and emotionally with this condition.