How many data could the uart in Pynq ardunio read in one time?

Hello ,I use the uart in the ardunio of PYNQ Z1 and Z2.
The python code is like this “lib.uart_read(device,number,len(number))”.
It shows that the length of data should be configured by user.

My problem is how long could the length of data be(see len(number))?

I try to make the length of number be very long, It seems some thing wrong. So I think there is a restrict of the length of data for uart in Pynq’s Ardunio to receive.