Due to mainland China's political, economic and strategic power it has successfully limited Taiwan's submarine force to just two operational submarines. This has prompted Taiwan to retain two aging Guppy submarines officially on the books to notionally boost its submarine force to four.
Taiwan's two ex US Guppy Class submarines are Hai Shih and "Hai Pao". They are only used for training (see Taiwanese Navy website). The two Guppy's were launched in the US in the mid 1940s then transferred to Taiwan in 1973. Their age makes them unsafe.
Taiwan's two more modern Chien Lung ("Sea Dragon") class submarines were built in the Netherlands. They include the Hai Lung, commissioned in the Taiwanese Navy in 1987 and Hai Hu commissioned in 1988. They are modified versions of the Dutch Navy's Zwaardvis class submarine. At 26 years old they have almost passed the usual 30 year "use by date" where rust, corrosion and structural weaknesses become increasingly serious.
In anticipation of Taiwan's submarines becoming too old, in 2001 the US Bush Administration promised to sell Taiwan eight diesel-electric submarines. The fact that the US has not built diesel-electric submarines in decades and did not intend to build any more rapidly prevented the US from keeping its promise.
Mainland China has pressured all diesel-electric sub building nations against supplying submarines to Taiwan. Diesel-electric submarine building nations have also been unable or unwilling to sell eight submarines to the US for transferring the submarines onto Taiwan.
Perhaps as a partial measure to increase the effectiveness of Taiwan's Chien Lung class submarines in late 2013 or early 2014 the US supplied Taiwan with some Harpoon anti-ship missiles which can be launched from the torpedo tubes of those two submarines. see http://www.presstv.ir/detail/2014/01/08/344401/taiwan-receives-us-submarine-missiles/ .
Earlier, in March 2013, the US supplied Taiwan with two upgraded early warning aircraft from the United States. The aircraft are all-weather early warning and control system platforms which in limited respects perform some early warning roles of submarines.
Comment: Given Australia has been able to build six large conventional submarines Taiwan (with a population and economy very similar in size to Australia) should be able to build eight medium sized submarines.
On Taiwan's future plans to build its own fleet of eight modern submarines Taiwan's Want China Times, January 11, 2014, reported that: