There are many power limits in the transmission line: thermal, stability, voltage drop and so on.

I think what you're thinking is the thermal limit...If you know the ampacity (maximum current it can carry) of a transmission line say 1000A typical and the voltage level at which it is connected say 132 kV,

you can then calculate the maximum power transfer limit of the line (Thermal) as P=sqrt(3)*V*I = 1.732*132*1000* 1000= 228 MW. (considering power factor as unity)

But if it is a long transmission line, the limiting factor could be stability limit at which case P=(V^2/X)* sin(delta)

Let's say, the same line considered above has a reactance of 0.3 ohm/km and a length of 300km, then P= (132^2/(0.3*300))*sin(30) (Taking delta of 30 degrees)

P= 96.8 MW

In short, if it is a short transmission line, the thermal limit governs the power transfer capability, and if it's a long transmission line, the stability limit governs the maximum power transfer capability.

Adding the already nice and complete information provided by Samundra Gurung , practically you can estimate the limit of power flow on a line from the SIL and line length. Refer to the link below.

https://neos-guide.org/sites/default/files/line_flow_approximation.pdf

Maximum value of power flow [kW] of transmission line by knowing R & X, if capacitance is ignored, may be evaluated from the expression

P = (E V sin delta)/ (X - R tan phi), where E, V are sending-end and receiving -end voltages, delta is the angle between these two voltages, and phi is the load power factor angle. With E, V,, X, R, phi remaining almost constant when delta approaches ninety degrees, obviously there will be maximum power flow.

The basis for knowing the value of the maximum power flow of a transmission line is to know the value of the surge impedance.

Surge Impedance Loading is a very essential parameter when it comes to the study of power systems as it is used in the prediction of the maximum loading capacity of transmission lines.