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An approximation of the effective number of bets constituents is the inverse of the Herfindahl-Hirschman Index (HHI) of the percentage weights in a given portfolio.

LaTeX Math Block
ENBENC_i = \frac{1}{HHI_i}

for any portfolio 

LaTeX Math Inline


In practice, an equally weighted index or portfolio has an ENB ENC equal to the number of constituents while a value weighted index or portfolio has a lower ENB ENC since some constituents contribute more than others to the index or portfolio performance.