This is a number pattern sequence. Looking at the structure: 2 (1 digit), 122 (3 digits), 12322 (5 digits) - the pattern is building palindromic numbers around 12. The sequence shows palindromes with increasing length: 2, 122, 12322, 1234322, 123454322. The missing number is 1234322 which continues the pattern of building palindromes with sequential digits.