Submarine Trends since 1990 - Actually fewer nuclear.

The diagram/map above is "Changes in the global submarine market since 1990" in The Military Balance 2015 Chapter 2: Comparative defence statistics, pp: 21-28. (Courtesy the International Institute for Strategic Studies (IISS) London, Febuary 2015).
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The biggest changes since 1990 have been:

-  far fewer US + Russian submarines overall by 2014, along with  

-  far fewer nuclear submarines worldwide.

United States

Under the Reagan Administration 1981-1989 there was very rapid US submarine building in pursuit of the 600 ship navy. During the Reagan years alone this included completion of 11 Ohio-class SSBNs and 28 Los Angeles-class SSNs. This rapid building program was in addition to high pre-existing numbers. So by 1990 the US had 125 nuclear = 36 SSBNs + 89 SSNs). No operational US SSKs.

The reduction in the Russian threat 1990-2010 permited a gradual decline in US numbers under the "peace dividend". From 125 there has been a steady decline in SSBN and SSN categories - though a  small rise of 4 in SSGNs.  

Soviet Union/Russia

Architect of the Soviet Navy Fleet Admiral Gorshkov believed that "quantity had a quality of its own" meaning high tempo, post WWII building of nuclear and conventional subs, with very few subs retired. By 1990 the Soviet Navy consisted of a large (Soviet Empire breaking) total of 273  submarines including: 

-  63 SSBNs

-  72 SSGNs

-  64 SSNs

-  65 SSKs

-    9 “auxiliary” (mothership and testbed)

Lack of money for crews, operations, maintenance and new subs led to the decommissioning of 200  Soviet/Russian nuclear submarines in the early 1990s.

Other Countries

In the 1990s many other countries slowed submarine procurement as part of the peace dividend. But regional tensions and some technical improvements have maintained or increased submarine numbers. Rising submarine aspirations of China and North Korea, from a very low base, have substantially added to the numbers of conventional subs in Asia.

Regional Tensions

Have included:

Turkey vs Greece

Israel vs Arab countries vs Iran

India vs Pakistan

India vs China

China vs all surrounding powers (Russia, US, Japan, South Korea, India)

North Korea vs all surrounding countries (Japan, South Korea, US, arguably China and Russia)

Vietnam vs China

Japanese and South Korean distrust (maintaining numbers).

Japan vs (Russia, China and North Korea)

Russia vs (all nuclear powers and Japan)

Southeast Asian nations distrust of each other and China

Latin America distrust and anti-drug smuggling

Business Opportunities 

Arms companies making submarines an essential naval component and political prestige item in all regions.

Some middlemen in some countries seeing the high costs of submarines as an opportunity to seek  "commissions" to provide necessary permissions.

Technological Improvements Leading to Gradual Arms Race

-  Several countries seeing subs as platforms for nuclear ballistic missiles. Ballistic include India, 

   China and North Korea. Nuclear cruise include Israel, Pakistan and North Korea.

-  SSNs, SSGNs and SSKs as platforms for "carrier killing" anti-ship missiles

-  SSBNs as second nuclear strike platforms and for some countries (like the UK) the only national
   nuclear weapon platform

-  Ability of SSNs, SSGNs to launch conventional warhead land attack cruise missiles used by the

   USN and UKRN since the 1990s and by Russia since 2015. Land attack cruise missiles are

   increasingly envisaged by some navies in Europe, Vietnam and maybe Indonesia and Australia.

Air independent propulsion (AIP), nuclear propulsion, and Lithium-ion Batteries have made little impact on overall numbers of submarines compared to the numbers of plain diesel-electic submarines operating in the late 1940s and 1950s. But the size and firepower of nuclear submarines is much greater than diesel-electic submarines.

Pete