I am trying to limit the response to have 9 numbers for ID

[Kal_Lam]

Maybe you could do the same as outlined in the image shared below:

In the survey tab of your xlsform:

Data entry screen as seen in Enketo: when a wrong entry is made

Data entry screen as seen in Enketo: when a correct entry is made

Reference xlsform:

Number (Regex).xlsx (10.2 KB)