What's the highest overclock you can do on a 2gb Geforce gtx 960?
That's too generic of a question. It depends on a lot of factors, including the air cooling capabilities of PC case, the GPU OEM's cooling performance, and like with trying to overclock a CPU, how good the GPU chip is itself. Generally if you buy a factory overclocked card, it will have more overclock headroom due to the OEM binning the highest quality GPU chips for their overclocked cards.
With that said, here's an example of what can be obtained (a factory overclocked example):
What's the highest overclock you can do on a 2gb Geforce gtx 960?
That's too generic of a question. It depends on a lot of factors, including the air cooling capabilities of PC case, the GPU OEM's cooling performance, and like with trying to overclock a CPU, how good the GPU chip is itself. Generally if you buy a factory overclocked card, it will have more overclock headroom due to the OEM binning the highest quality GPU chips for their overclocked cards.
With that said, here's an example of what can be obtained (a factory overclocked example):