Any number of slave channels can be paired to the same master channel, in this case the ANT USB having one slave channel paired to the Mio Fuse's master channel. The same device however can only have 1 slave channel paired to external master channel at a time as it's only a single timeslot it's paired to...
If you are referring to how many master channels or heart rate devices a single ANT USB stick can listen to, you have 3 options. If you just use slave channels on the ANT USB device, then it's 8 (or 15 if you use an N5 module). If you use continuous scan mode then it's approximately 50 (200/4Hz/HR device). If you use background scanning then it is also ~50, but only for the ANT-USBm as it supports high duty search.