In both style guides, numbers that begin a sentence should be written out as words and Chicago specifies that, if this is an unwieldy solution, then the sentence can be rephrased. (Number Style 101) provides a useful guide, which goes into greater detail on good style for numbers in general documents.
Recasting the phrase is generally the better option.
When the precise value must be conveyed, the decimal or percent form is preferred.
I would write "Let two firms engaged in oligopolistic competition" rather than "let 2 firms..." (this is "counting the cardinality of a collection").
I would also write "Let fifty firms..." rather then "Let 50 firms" But, I would write "the value of this parameter is set to 6", rather than "... Many style manuals will have specific requirements regarding when to express a number as numerals or in words.