Each investigator can investigate once per day.
How many pieces of evidence can an investigator analyze in a day?
Does returning a piece of evidence consume stamps and envelopes? Is this a game engine?
Investigators can interpret Evidence that matches their specialism as often as they like. The once-per-day limit is only on investigation, i.e. rolling on the Investigation Table.
The limit is also not a strict rule. If you wish to play by electronic mail or instant message, I don't recommend following it.
Whenever Investigators wish to communicate with each other, they must use stamps. If you are using a method of communication that does not require stamps, please keep track of how many you would have remaining were you using stamps.