calculate the RSet resistor value for MAX7221 IC
It is all on page 10 of the data sheet:-
The current per segment is approximately 100 times
the current in ISET. To select RSET, see Table 11. The
MAX7219/MAX7221's maximum recommended segment
current is 40mA. For segment current levels
above these levels, external digit drivers will be needed.
In this application, the MAX7219/MAX7221 serve
only as controllers for other high-current drivers or transistors.
Therefore, to conserve power, use RSET = 47k[ch937]
when using external current sources as segment drivers.
how should I drive 7-segment displays with that same IC with 9 volts
You can't use this IC to drive a matrix at 9V it is only designed for a 5V system. So you can not use this chip with this display.
One thing that I found out is that I could use MAX394 with MAX7221
You found out wrong this chip is of no use to you in this application.