The U.S. government released a DIA (Defense Intelligence Agency) analysis of Taiwan's air power situation. A shortage of new aircraft, and spare parts, have left the Taiwan Air Force with less than 300 combat aircraft (out of 400 they own) they could depend on in a war with China. The details are pretty ugly. Only about half of the 60 elderly F-5s are available for service, and the locally made IDF is, and this is no secret, a second rate F-16 replacement. The 56 French Mirage 2000-5 are expensive, and manpower intensive, to maintain.
The Taiwan Air Force is considering retiring the older, more difficult to maintain, aircraft, and put more resources into the more recent models. Taiwan wants to upgrade its 146 F-16A Block 20 fighters, which are some of the oldest F-16s still in service. Taiwan has on order 66 F-16C block 50/52 fighters, a sale which has been blocked by local politics, and Chinese protests, for years.
The DIA report also pointed out the poor layout and positioning of Taiwanese air bases. The Chinese have had ballistic missiles (now over a thousand) aimed at Taiwan for over a decade, yet Taiwan has done little to harden its air bases to deal with getting hit by these missiles. The Chinese missiles carry one ton or half ton conventional (high explosive or cluster bomb) warheads, and were expected to be used to try and cripple Taiwanese air force and navy, as well as attacking headquarters and communications targets.
China denounced the DIA study as part of a plan to justify more American weapons for Taiwan.