Love the Panel driver and I am using it over on my https://github.com/Qrome/marquee-scroller Scrolling Weather Clock.
My software supports the chaining of several of the Matrix displays. We found that there is a limit with the current configuration of up to 16 displays because of the int8_t on line 106 of the Max72xxPanel.cpp file. When we changed it to be an int16_t then we were able to use 20 or more chained together.
Maybe you could consider updating line 106 to int16_t so that it isn't limited to just 16 displays.
Thanks!