Solution: Number Line

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The numbers can be arranged to form a number line from "1" to "26", with a few numbers missing:

1 234 578 910 111 213 141 617 192 122 232 426
1 2 3 4 5 _ 7 8 9 10 11 12 13 14 _ 16 17 _ 19 _ 21 22 23 24 _ 26

The number line represents the 26 letters of the alphabet and the missing letters are F O R T Y, which spell "FORTY", the missing number and the solution to this puzzle.