LAUNCHXL-CC1312R1: 6lowpan mesh ti 15.4: uIP limiting number of routes

I would like to limit the number of routes that join the network to 15 even if there are more and I have a problem.
I decreased “UIP_CONF_DS6_ROUTE_NBU” in the "config-uip-public.h" file to 15 instead of 200 routes but what happened is:
1- More than 15 nodes joined the network but I want only 15 nodes to join.
2- The RPL table in the root coordinator only stores 15 routes and if there is an incoming DAO message from a new node,
the RPL removes the old one and stores the new one.
How can I limit the network into a specified number and in the same time the other nodes search for other root coordinators to join?