How does the US not know how many they have?
I’d guess that our intel people actually do know, but choose not to make it public.
I feel sure that our military has all the information they desire on Singapore’s aircraft that are US sourced planes. They partner with us in training and maintaining on all these aircraft and have training groups at a number of US AF bases.
With the USAF they participate fully in exercises and positioning of their forces. What they don’t do is publish everything that liberal media would like as they (and we) don’t want the Chinese and others to know all of their capabilities for certain.
This article is written as though this is a mystery or something is amiss. It is only a mystery to those who don’t need the information.