Selecting the correct Altera CPLD for LED signal decodings

You will at least need a 5M80Z because with 48 outputs you need 48 registers and the 5M40Z only has 40.

If you want 'no flashing while the outputs are updated' you need another 48 buffer-registers and thus at least a 5M160Z.

If your serial input is a 'true' serial you may need a bigger device still as you would have to implement a baudrate generator and such.

If you can do some bit-banging, i.e. a data signal and a strobe, the 5M80Z/5M160Z will very probably be OK.