I can't think of a way aside from cron every X seconds. I'd dump it into a csv file with just the timestamp and the number of calls. E.g.
#!/bin/sh
if [ ! -f /var/log/activecalls.csv ]; then
echo "Timestamp,Calls" > /var/log/activecalls.csv
fi
DateTime=`date "+%Y%m%d %H:%M:%S"`
echo -ne "\n$DateTime," >> /var/log/activecalls.csv
asterisk -x 'core show channels' | grep 'active calls' | cut -d " " -f1
Which SHOULD create a nice csv for you...