Hi Yong,
The nRF5 supports up to 15 channels in its ANT software stack. Changing the MAX_ANT_CHANNEL parameter will not change this limitation.
Depending on what your use-case is, you may want to consider using a continuous scan instead. So instead of tracking 60 individual channels, your program will listen to all ANT+ HRMs in proximity. If you enable extended assignment, you'll be able to differentiate the different HRMs by means of their extended device number.
Hope this helps,
BK.