Is the id numeric or is it alphanumeric?
the API returns a string
→ Reference | Twitch Developers
It should be treated as a string.
The actual format is not documented and whilst it could change at any time it’s not expected to and so far has been numeric.
For database stuff you should store it as a string
Should I validate it as alphanumeric?
Why validate it at all?
It’s data returned from the API it should never be wrong
make sense 
thx