On the keeping number of social security files

Do you mean the social security payment record or your personnel file?

If it is a social security record, you can call the social security service phone of your previous work place and enter your ID number to check whether the social security is local. Generally speaking, an individual's social security registration number is your ID number;

If it is a personnel file, it depends on which unit your personnel file was hung in at that time. Generally, when you leave a unit, the unit will not handle your files. It has always been handled by a personnel agency entrusted by an individual or entrusted to the unit for collective custody. If you find it, you can transfer it after paying the escrow fee.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.