From May-September, this service runs fast to Plymouth (with a pickup only at Exeter), with an additional service 2 minutes behind it calling at Reading, Taunton and further stops to Plymouth, to manage increased demand. Outside of this period it is not as in demand, however the Monday after the London Marathon it was very busy (something I found out the hard way by travelling on it), to the point the train manager made customers aware of GWR's No Seat compensation claim option.
I would imagine the best trains to get from Totnes regarding crowd levels are Plymouth starters, as Totnes is the first call after this. While you'll get Cornish passengers changing at Plymouth to board a Plymouth-Paddington service, far more will prefer the direct services. Also late-afternoon tends to be quieter, as those travelling afar will have set off in the morning or early afternoon.