Infrastructure is "plot thick"
There's no established rules or ratio for how much Infrastructure is in a given area; thematically, it may work for the chronicle for the location to be a hotbed of many small installations; or perhaps a giant one. That being said, a truly barren and devoid place, a blind spot to the GM, would represent a hot commodity that many Demons would be willing to fight over, growing louder and louder until they get noticed.
From the GM's perceptive: Infrastructure often daisy-chains, so it makes sense to have some clustered ; but on the same hand, if one is compromised, it'd be inconvenient for a large project to cascade into failure.
From the ST's perspective, Infrastructure is both "crisis" and "opportunity", so it makes sense to have the right amount for the chronicle tone, troupe size, etc.
But, if you still want an official metric, the "setting city" of DtD is Seattle, Washington. Between DtD's Appendix One, the "How an Angel Dies" chronicle included in DtD, and the Splintered City: Seattle sourcebook, there's about 12 pieces of Infrastructure fleshed out. Ironically, these include the FBI office and a self-replicating chain of coffee shops.
Now, assuming there are about 200 Starbucks in the Seattle metro area, one could fiat a good number of those into Von Neumann's, but let's count those collectively as 1.
Current Wikipedia stats gives Seattle's footprint as 142.5 square miles, 725K residents (3,87 million for entire metro area) which gives roughly:
- 1 Infrastructure per 12 square miles
- 1 Infrastructure per 60K residents
- 1 Infrastructure per 323K residents (entire Metro area)
Off course, the caveat is that Seattle is not going according to plan, what with all the time splinters. Maybe the Infrastructure was too dense, and the GM has since spread more out... or maybe, because Seattle has "fallen", it's actually less dense, because the GM doesn't trust it anymore.