AutoCAD LT

[#1]

I have combined all the licenses of our company to one SN.

but all old SN still can be used.

is that normal?

[#2]

You need to contact an Autodesk reseller to discuss licensing.

[#3]

The system should eventually expire those old serial numbers and prevent them from being activated.  You should get an error stating that the license serial number is invalid.  Granted, this will not affect any previously installed and activated software using that serial number.  However, subsequent reinstalls should no longer work.  In any case, for licensing compliance, you should only use the number of licenses that you own.  If you have any questions, contact your Reseller as Dean advised.