SANGIS publishes a geographic file of the locations of "drain structures", which appear to be the entrances to storm drains. There are a lot of these; here is a map of all of them in san diego, colored by the community the drain is in:

A raw count of drains will depend on the size of the community, so here are the san diego communities that are larger than 2 sq kilometers, ranked from the smallest number of drains per sq km.
name DRAIN area count_per_area
---------------------------- ------- -------- ----------------
TIJUANA RIVER VALLEY 22 14.6227 1.50451
SAN PASQUAL 72 43.1926 1.66695
MILITARY FACILITIES 262 92.1037 2.84462
LOS PENASQUITOS CANYON 52 4.94249 10.521
PACIFIC HIGHLANDS RANCH 140 10.7745 12.9936
MISSION BAY PARK 383 17.4868 21.9022
DEL MAR MESA 214 8.50495 25.1618
TIERRASANTA 929 29.5284 31.4613
NCFUA SUBAREA II 137 3.45853 39.6122
RESERVE 150 3.31251 45.2829
SERRA MESA 453 9.00942 50.2807
BLACK MOUNTAIN RANCH 1065 20.7387 51.3532
OTAY MESA-NESTOR 1129 21.8699 51.6236
FAIRBANKS RANCH COUNTRY CLUB 170 3.2103 52.9545
NAVAJO 1995 37.0283 53.8777