While there are "gears" in a Prius, in the reference that most people would be asking this question, there are actually NONE. The Prius(all 3.5 generations as of 2010) uses a planetary differential with the engine at one point and a Motor/Generator(MG) at another point and another MG which is attached to the wheels at another. The MG's can each also supply power or drag in either direction.
What this means is that there are no clutches to disconnect any point, but the Prius can be completely stopped or moving at speed while the engine is completely shut off or spinning. It simply adjusts the balance by way of speed, direction and drag of the 2 MG's through the planetary differential.
The 2001 Toyota Prius has 16 valves.
The 2011 Toyota Prius has 16 valves.
The 2010 Toyota Prius has 16 valves.
The 2002 Toyota Prius has 16 valves.
The 2006 Toyota Prius has 16 valves.
The 2005 Toyota Prius has 16 valves.
The 2013 Toyota Prius has 16 valves.
The 2004 Toyota Prius has 16 valves.
The 2012 Toyota Prius has 16 valves.
The 2003 Toyota Prius has 16 valves.
The 2014 Toyota Prius has 16 valves.
The 2008 Toyota Prius has 16 valves.