RRM will maintain the assigned resource details, say example ENB has 50Mbps of resource in 10mhz Bw. If NW configures default bearer as GBR say example 1mbps of GBR rate then NW can assign only max of 50 GBR default bearer because it has only 50mbps of guaranteed bit rate. Once you assigned GBR bearer with some GBR rate ,ENB should not give that resource to any one else(That is the concept of GBR bearer even if it not utilized by that UE), in case of N-GBR bearer there is no gaurenteed bit rate so NW can admit more than 50 UE(in this case). That is the reason NW always configures N-GBR as default bearers.
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