Common Sequence Number across all forms (serial number)

Hi,
I have 5 different form on my website.
I am using special mail tag to get serial number in the emails.

I am using the serial number a unique identifier and feeding it into the CRM.

Now the problem i am facing is, each form has it own sequence number which is overlapping with sequence numbers from other forms.

I want a common and continuous sequence across all forms so there are no duplicates.

How to achieve it.

Thanks.