I originally got a few of for my niece who was in drama class after the first few she wanted all I could find so eventually I managed to come up with with 26.

26 Acting Dialects from David Alan Stern includes the following:

Acting With A Boston Accent
Acting With A British Cockney Accent
Acting With A British North Colony Accent
Acting With A Down East New England Accent
Acting With A French Dialect Instruction
Acting With A German Dialect Instruction
Acting With A Midwest Farm Ranch Accent
Acting With A New York City Accent
Acting With A Norweigian & Swedish Accent
Acting With A Persian (Farsi) Accent
Acting With A Polish Accent
Acting With A Russian Accent
Acting With A Scottish Accent
Acting With A Spanish Dialect Instruction
Acting With A Standard British Accent
Acting With A West Indian & Black Africa Accent
Acting With A Yiddish Accent
Acting With An American Southern Accent
Acting With An Arabic Accent
Acting With An Austrailian Accent
Acting With An Chicago Accent
Acting With An Irish Accent
Acting With An Italian Accent
Acting With An Texas Accent
Acting with an Upper Class New England Accent
Dialect Monologues Vol II
Dialect Monologues Vol I