It depends on location - this for the spaces between TV broadcasters - areas with lots of TV stations will have less than areas with few.
The frequency band for TV covers several frequency bands:
--59-88Mhz (88Mhz is where FM radio begins)
--174Mhz to 216Mhz
--Channels 14 - 51 (52 and above are lost to the DTV transition)
470Mhz to 698Mhz
Each TV channel occupies 6Mhz of spectrum bandwidth (Digital or Analog) minus additional channels that broadcasts use between towers and from remote trucks to studio to transmit live - lets ASSUME they take an additional 3-5 channels for each station for off-site. Each "On Location" and studio to tower link would eat these up.
Remember that studios are not often near the broadcast towers (due to the towers needing to be at high elevations) so they often broadcast the signal to the tower and it is retransmitted on the true station for service.
Even with an average 9 TV stations per area this could easily equal 36 or so channels actually setup for use.
36*6=216Mhz taken just for TV use. That is most of the ~290Mhz that is available between VHF and UHF. WHitespace devices will most likely stay in the UHF potion which trims down the spectrum to allowing just around 12mhz for a moderately populated area.
In contrast Wifi has around 84Mhz available and many tech notes suggest only running 3 "channels" (1,6,11) so that the devices will not step on each other.
5.8Ghz has 125Mhz for use and there is 26Mhz of 900Mhz that may be used. Lower frequencies allows for greater range, but you give up data rates...
If you can't tell I'm not a fan - I'm not seeing the upside to these free "whitespaces".