How to create a circuit to drive a 3 Phase BLDC Motor?

As the question stated. What I saw online was this, but I couldn't exactly understand why I needed 6 MOSFETs and not 3.

https://www.digikey.at/de/articles/how-to-power-an...

The circuit I had created was actually very similar to this. I have 3 IRF840 MOSFETs for the application, because it is a large motor. For this, I have connected an Arduino and programmed it basically to emulate a PWM with each output pin being on at their respective time for 20 ms (since I need 50 Hz as PWM). 

Now, since IRF840 requires a gate voltage of at least 10V, I'm going to connect low current/low voltage PNP transistors in cascade with each Power MOSFET, operating at 12V. So, basically, Arduino to control PNP transistors to Power MOSFETs. 

Below you'll see the circuit I found in the website I linked above. Now, here is my question. If I am to control this circuit with an Arduino, what exactly is the frequency that I need to use for each PWM input in the figure? Should it be 10 ms or 20 ms if I am to achieve 50 Hz, because from what I understood, one set of MOSFETs is affecting the North Magnetic pole whilst the other set of MOSFETs is affecting the South Magnetic Pole. Again, I can easily program this, but I just need to make sure. Note that I only need to run the motor on a single speed, so no need to add additional circuitry for dimming. 

Thank you all for your answers :)