How many files are created by default using WiredTiger storage engine?
Yes, WiredTiger Engine creates one file per collection and per index.

Files are the part of storage layer that is being maintained by the Storage Engine part. you can locate the created files in the dbpath specified in the configuration file used to start the server instance. For example in case if your underlying OS is ubuntu the default config file path is:

/etc/mongod.conf


In case of WiredTiger engine when you create any collection, then you will be able to find new files like:

collection-22--9037026691096837958.wt
index-23--9037026691096837958.wt


here numbers 22,23 indicates the unique identity of collection and index to the storage engine and being maintained withing the storage engine's metadata.

in last part of your question about threshold, you may be interested in storage capacity limits, so you can consider following these links:

https://docs.mongodb.com/manual/reference/limits/
https://stackoverflow.com/questions/4327723/mongodb-limit-storage-size