Network Efficiency With NAT Issues

Suppose I have a group of 500 independently operated nodes, but for various reasons, NAT etc…, some are only client nodes, not server. Of course, the most efficient would be if 100% of the nodes are in server mode. But, given that the nodes are operated by independent parties, that goal is probably too ambitious. So what I need to know is, what percentage of client mode nodes is acceptable and at what point does that become too high and not enough server nodes remain to maintain an efficient network.