I wrote some code which can send an arbitrary number of data from Arduino to Pd, without needing to define that number before hand (you only need to define the maximum number of data).
Dunno if someone has done this before, or if it can be done in a better way. If you know of one, please let me know.
You can find it here https://github.com/alexandros301/Arduino_Pd_arbitrary_num_of_data
Cheers