$f(x)=\frac{250}{1+7 e^{-3 x}}$. Find the carrying capacity.

Step 1 ：The carrying capacity of a function is the maximum output value of the function. In this case, the function is a logistic growth function, which has the form \(f(x) = \frac{C}{1 + Ae^{-Bx}}\), where \(C\) is the carrying capacity.

Step 2 ：Therefore, the carrying capacity of this function is 250.

Step 3 ：Final Answer: The carrying capacity is \(\boxed{250}\).