How many ADS8413 connect in cascade mode?

It is dependent upon the frequency of the serial clock and by extension the limits of your controller. It is roughly described by:

TIMING DIAGRAMS FOR DAISY CHAIN OPERATION
The conversion speed for n devices in the chain must be selected such that:
1/conversion speed > read startup delay + n*(data frame duration) + td16
Read startup delay = 10 ns + (td19 - td4) + td12 + 2/fCLK
Data frame duration = 16/fCLK

This just says that you have to read out all of the data from all of the devices serially during the time for another conversion to be completed.