If you look at real network traffic flowing through e.g. a mobile operator's core network, you'll find that for 1Gbps of traffic there will be anywhere from 200k to 500k open TCP connections (depending a bit on the traffic mix, and on what idle connection thresholds you use for the analysis). So if you're making TCP-aware network appliances targeting a 10Gbps traffic rate, it's not a bad idea to plan for simultaneous 10M connections. You might get away with a factor of 2 or 5 less, but probably not with a factor of 10. 150k connections would be a joke.