Heat sink calculation for TDA7376B
There appears to be a mistake in the graph, which is why you might be having trouble interpreting it. The efficiency \$\eta\$ and Ptot annotations are swapped. Say you want 10W out, the power Ptot is 24W and the efficiency is about 45%. From either of those numbers, you know the amplifier IC will be dissipating ~13W. 

So if you allow a maximum Tj of 130°C and allow a maximum air temperature of 45°C at the heatsink you would need 8.5°C/W J-A. The device itself eats up 1.8 leaving 6.7°C/W for the heatsink. You can look up suitable heatsinks- the amplifier package is relatively wide. 

Whether 10W (continuous, much higher for brief bursts) is enough or way too much is going to depend on you, your speakers, the source material etc. 

The absolute maximum operating junction temperature is 150°C but that is very, very hot for a plastic packaged device. It's better to be more conservative.