NOTE: This is a count of full-power TV stations only.
The Empire State Building has 12: WCBS, WNBC, WNYW, WABC, WWOR, WPIX, WNET, WNYE, WPXN, WXTV, WNJU, and WFUT.
Willis Tower also has 12 (split between its two masts) :WBBM, WMAQ, WLS, WGN, WTTW, WCPX, WJYS, WCIU, WXFT, WPWR, WFLD, and WSNS (The John Hancock Center has only 2 TV stations, WYCC and WGBO).
Sutro Tower, which was highlighted on last week's Tower Site of the Week, has 11 TV signals emanating from it: KGO, KBCW, KPIX, KRON, KTVU, KOFY, KQED, KMTP, KCNS, KCSM, and KFSF.
Washington's Tenleytown Tower supports 5 signals: WJLA, WUSA, WETA, WHUT, and WPXW.
Raleigh's WRAL Digital Candleabra also supports 5 signals: WRAL, WNCN, WLFL, WRDC, and WRAZ.
The East Tower at Atlanta's Richland antenna site, which appears to hold the most full-power TV signals in the "ATL", has only 3:WPCH, WATL, and WUVG.
Feel free to add others to the list...