Discovery is done via the USM Discovery Wizard, and not snmpcollector. snmpcollector queries the discovery server for the snmp devices, and then creates profiles for them.
How many devices were discovered and a list of them ?
You can use the metrics page Franklin provided to get that.
Also you can create a group in USM do display all snmp devices.
How many devices weren´t discovered and why and a list of them ?
Consider a discovery can use a range of IP addresses, there would be no way to know how many devices are out there. What you can do is in Discovery Wizard create one scope for every device, and then see which worked.
And a list of thresholds configured for a template.
Only way I know of is to check the templates and see what has been set.