System number can be found by any of the following ways,
1. In SAP GUI (SAP Logon), click on the SID and then click on change items,
In connection page tab, at last in Grayed color you can see the system number of that particular SID.
2. You can see the system number at the /usr/sap/<sid>/DVEBMGSXX where XX is the system number.
Also it can be found from system Instance profile <SID>_DVEBMGSXX_<host>
3. Execute transaction SM51, status of instances can be seen, in that you can find the naming convention as below:
<hostname>_<SID>_<instance/system no.,>
4. You can get the System number by going to Tcode - slicense.