It depends on your series string total voltage and the minimum acceptable MPPT voltage for your system.
It also depends on condition of bypass diodes in panels.
Easiest to assume any panel with some shading will totally drop out and will have a three-diode voltage drop, say -3v drop. If the sum of the illuminated series panel mppt voltage - (3v x number of shaded panels) is still greater than your minimum MPPT system voltage the string will still produce output at a reduced power.
I mentioned bypass diode condition because often manufacturers put insufficient poor quality bypass diodes in the panel. When bypass diode is bypassing it carries panel current. This current x diode voltage drop is heating on the diode. If you have a 9 amp Imp panel and diode has 0.9v to 1.1v drop it will be dissipating 10 watts of heat. A small pellet bypass diode will cook and self destruct.
A cheap poor quality panel may put diodes in parallel under the assumption it can carry more current. This is bad engineering as diode with lowest voltage drop will take most of current. As diode gets hot its voltage drop reduces and it takes even more current so parallel diodes is a downward spiral to full destruction.
A good panel has large bypass diodes packaged in something like a power transistor case with some heat sinking provided.
A bypass diode is usually put across every down and back cell row pair in a panel and are located in panel junction box. For example, if panel is 10 cells long x 6 cells wide (60 cell panel) it will have three bypass diodes in junction box for three down and back row pairs.
If one bypass diode on any shaded section burns out to open state you will get zero output from total series array of panels.
A partially shaded panel can produce some output but it is difficult to predict how much.