The digits at the end of the hot bucket directories are sequential numbers. Hot buckets are rolled to warm (db_*) based on index configuration parameters, or when you stop/restart splunk. So those hot bucket names change all the time as new data comes in.
defaultdb is (by default) mapped to the 'main' index. If you don't ingest any data here, you won't have hot buckets.
I would recommend you use the Splunk Sizing Tool to figure out what your storage requirements are.
Select your daily data volume, retention settings, etc. and it will give you an estimate on per-indexer and total data storage needs for HOT/WARM and COLD volumes.