I see the mind trick as more of a gradient difficulty depending on several factors, which include but are not limited to the weak-mindedness of the subject(s), the number of subjects, and how much you want the subjects to deviate from their intentions. On a scale of Not Really Tricky to Impossibly Tricky:
For Obi-wan vs the stormtroopers, it probably would have been easy to sway their minds and also to have them simply overlook a mildly suspicious pair of droids. However since it’s a group of Stormtroopers, he would have had to mind trick all of them to some extent for it to work. Moderately Tricky
The novel suggests that Obi Wan only had to mind whammy the lead trooper. IIRC, Luke dares to look back after they pass the checkpoint and the subordinate troopers are arguing among themselves.