Ignition How many devices can be attached to a channel, how many signal points can be accessed by a channel, and how many signal points can be accessed by a device
If you mean how many tags can be accessed by how many device connections, the answer to both is there is unlimited.
Your only limit is hardware and networking constraints.
Ignition doesn't use the "channel" term itself, though some external OPC UA servers do (like Kepware). Such external servers may have their own limits.