User blog comment:Suparockr/List of Plotholes throughout the series/@comment-4950455-20120725013903/@comment-2250460-20120725014149

Cerberus was shown to have large numbers of ships even before Mass Effect 3 i.e. Mass Effect:Invasion.

You have to remember Cerberus has existed for awhile now and has Billions of Credits to spend, it's likely this to factors resulted in the large fleet.