If you're asking how much natural gas a 3/4 line can carry, the answer is quite a bit. In fact, a 3/4 line can carry up to 60,000 BTU/hr! Of course, this assumes that the line is properly sized and that there are no other factors that would restrict the flow of gas.
Now, if you're asking how many BTU can a 3/4 line deliver, that's a different story. The answer to that question depends on a number of factors, including the length of the line, the number of fittings, the type of gas being used, and the pressure at which the gas is being delivered.
In general, though, a 3/4 line can deliver up to 40,000 BTU/hr. So, if you're looking to install a gas line that can deliver a lot of BTU, a 3/4 line is a good option.