Forbes last year named 16 billionaires, but Ventures on Monday said its "on-the-ground knowledge" had helped it identify many more.
Of the 55, 20 are Nigerian, including several oil barons, while South Africa and Egypt boast nine and eight, respectively.
The richest man is Nigeria's Aliko Dangote with a fortune of $20.2 billion.
The magazine estimated the 55 billionaires' combined fortunes at $143.88 billion, an average of a $2.6 billion per person.
Ventures listed businessman Allan Gray as South Africa's richest man with a fortune of $8.5 billion.
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