Not only the number of arguments; also the type.

The commonest error when using printf in C51 - and one specifically addressed in the Manual section that I cited earlier - is to use a %d in the format string, but supply a char parameter:
The %d specifier requires an int value, which is 16 bits in C51; so you will obviously get rubbish if you give it a char - which is only 8 bits! This will leave the parameter-passing mechanism out-of-step, so all subsequent parameters are likely to also print as rubbish!