Which policy, rule or document
of RIPE describes that if ISP giving to customer more than one IP address then
those addresses have to/should be saved into RIPE database as independent
entry of inetnum?
You can find the relevant
information in http://www.ripe.net/ripe/docs/ripe-424.html#4:
"All assignments and allocations must be registered in the RIPE Database."
In your case, you have to register every network and provide information who it
is assigned to. Also look at http://www.ripe.net/ripe/docs/ripe-424.html#62,
which explains the difference between network infrastructure and end user
networks.