One at each end of the Sol => Ross 248 jump point, one at each end of the Sol => Procyon jump point, one at each end of the Ross 248 => Nexus jump point... that covers all the path to-and-from our colony worlds... and perhaps one at the near end of the Ross 248 => Groombridge 34 jump point. That's seven per year. Hold the rest in reserve for contingent use.
That sounds fine, and the 50 will last a while yet until a full time replacement becomes available.
Anyone who attacks Earth directly is probably in for a shock, unless their tech is just grotesquely higher than ours. When Charlie was closing in on Earth, he was in a cross-fire that could deliver 174 size-4 to size-8 missiles in the first salvo, without reloading. That should totally saturate his point defenses, allowing nearly half of the missiles to hit.
In my rule-of-thumb calculations, I count on his Point Defense taking out about half of our missiles... until it's saturated, when all the rest get through... after which about half of the missiles that penetrated his point defense should score hits.
That does sound very impressive, it would be interesting to see how it fares in combat. I had something to say but it slips my mind now.