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If you mean: 9, 19, 39, 69 then the next number in the sequence is 109
If you mean GOOGOL, the next number is one googol plus 1.
The Roman numeral XI represents the number 11 and the next number, 12, would be XII
An unknown number
Do you mean the next number or the sequence equation. And there isn't a solid patter, there's to much ambiguity to determine a logical next number.