sharepoint online – How to insert multiple values into one column separated by semicolumn

I don’t think you can validate this column using calculated formula to meet the requirements. Because, the length of the column value is dynamic, and none of the followings could be validated dynamically:

LEN((NOTIFICATION NO.))=9
MID((NOTIFICATION NO.),1,3)="500"
ISNUMBER((NOTIFICATION NO.)+0)

However, if we know the maximum number of semicolon-separated (;) values, then it is possible to construct part of formula using OR construct.

For example, assuming there are three values like you’ve mentioned 500123456;500567890;500765456

OR(LEN((NOTIFICATION NO.))=9, LEN((NOTIFICATION NO.))=19 , LEN((NOTIFICATION NO.))=29)

Also, functions like REPLACE() can’t be used to replace all semicolons to verify if the string contains only numbers. Need to repeat this function to a known number, and can’t be a dynamic value.

REPLACE((NOTIFICATION NO.), FIND(";",(NOTIFICATION NO.)),1, "+")
Output: 500123456+500567890;500765456

The goal is to check ISNUMBER(REPLACE((NOTIFICATION NO.), FIND(";",(NOTIFICATION NO.)),1, "+")+0) 

It would be better to use some other means like Power Automate or JavaScript to validate the column value.