How can I find a collector id ?

You can find this information in the "main.properties" file that can be found under "<Sumo_home>/config/creds directory.

For example:
$ cat /Sumo\ Logic\ Collector/config/creds/main.properties

#Auto generated.
#Mon Nov 26 16:42:17 PST 2018
obfuscated=true
apiId=151413262 . ---------> this apiId is the unique collector id
key=*******

Note that the ID after "apiId=" is a decimal representation of the collectorId. The collectorId that is displayed in the Collector logs or under <Sumo Logic Collector Install Dir>/logs/collector.log will be a Hexadecimal number representation of the same number. You will need a site like this link to determine the collector Id number that will be displayed in the collector logs. You will see a string like "Registered as collectorId=00000000906620E" where the hex number "906620E" is the equivalent of the decimal number "151413262" above