Seeing the frame rate in FS2020 can be very useful. Having an awareness of the frame rate can help you understand how different settings and tweaks can affect the overall performance of your simulator, and identify potential deficiencies in your hardware.
There are two ways to see the frame rate in Microsoft Flight Simulator:
Option 1: Developer Mode
First, go to the Options tab, then select General Options, then Developers. Set the developer mode to ON.

Next, in the [DevMode] bar, select “Options” and “Display FPS:

You can then observe the FPS readout in the upper right of your display. The large number at the top (in this case 58.5) indicates the current number of frames per second (FPS).

Option 2: Xbox Game Bar
This option requires that you have the Xbox Game Bar installed. By simply pressing “Windows Key + G” on your keyboard, you can see FPS, along with other resource utilization information.

Alternatives
If you prefer a different, more compact style of FPS display, there are alternatives, such as using Steam’s In-Game Overlay or NVIDIA GeForce Experience.