How to generate a unique numerical value for an alpha-numeric string?
If you don't limit the length, there are infinitely many alphanumeric strings. The number of even 128-bit numbers is finite, so there's no way to assign a unique 128-bit number to every possible string.

That said, if you don't mind a very small risk of collisions, a cryptographic hash function like SHA-2 or SHA-3 truncated to your desired output length should be fine, as long as that output length is sufficiently large.  With e.g. a 128-bit output size, the average number of strings you'd need to hash before observing even a single collision is around $2^{64}$, probably far more than your software will ever see.  If you're really feeling paranoid, bump the length up to 256 bits.

(A point I should probably clarify is that hash functions like SHA-2 and SHA-3 output a fixed-length block of bits.  While this output is often represented in hexadecimal for convenience, it's really just a large binary number and could just as well be displayed in any other base, e.g. in the plain old decimal system we humans like to use.)