1996: 151 new Pokémon
1997: (I won't bother with this one... let's just say 0 new Pokémon)
1998: (same as above)
1999: 100 new Pokémon
2000: 0 new Pokémon
2001: 0 new Pokémon, 1 new distribution
2002: 135 new Pokémon
2003: 0 new Pokémon, 1 new distribution
2004: 0 new Pokémon, 3 new forms, 1 new distribution
2005: 5 new Pokémon (spinoffs and anime)
2006: 102 new Pokémon, 1 new distribution
2007: 0 new Pokémon, 1 new distribution
2008: 0 new Pokémon, 2 new forms, 1 new distribution
2009: 0 new Pokémon, 1 new distribution
2010: 156 new Pokémon
2011: 0 new Pokémon, 1 new distribution
2012: 0 new Pokémon, 3 new forms, 1 new distribution
2013: 72 new Pokémon, 28 new MegaEvolutions, 1 new distribution (100 new designs)
2014: 20 new MegaEvolutions, 3 new forms, 1 new distribution (23 new designs)
2015: 0 new Pokémon, 4 new forms (anime), 1 new distribution (4 new designs)
2016: 81 new Pokémon, 18 new regional variants, 1 new distribution (99 new designs)
2017: 5 new Pokémon, 1 new distribution
2018: 2 new Pokémon, 1 new distribution
2019: 3 new Pokémon (so far)
1996-1998: 3 years, 151 new designs
1999-2001: 3 years, 100 new designs
2002-2005: 4 years, 143 new designs
2006-2009: 4 years, 104 new designs
2010-2012: 3 years, 159 new designs
2013-2015: 3 years, 123 new designs
2016-2018: 3 years, 104 new designs
They're not making less Pokémon, they're just revealing them differently. I expect them to reveal more and more Pokémon every year, and less Pokémon at the start of a new "generation", until it gets to the point that they have about 30 new Pokémon every year. Having exactly 35 new Pokémon every year would mean 105 new Pokémon every 3 years, or 140 new Pokémon every 4 years.