The easiest way is to use pybgpdumppybgpdump
The package includes a tool in the samples directory called simple.py which will summarise the announce/withdrawal count for a given MRT file:
2630 total messages with 12028 announced and 5666 withdrawn routes
If you're after an entire day's worth of data, you'll need to download all the 288 files of 5-minute intervals, and then hack simple.py to iterate through the list.