Assuming that you also have two tables called Trail and Obstacle and a relationship (TO) to the Obstacle table, then in the Trail table create a calculation field and insert the following formula:
result is a number and is unstored.
Let me be more specific. I have multiple trails with many obstacles. Each trail is numbered. I'm looking for an obstacle count for, let's say, trail 3.
We need to know the relationship.
If you have this relationship:
Trail::TrailID = Obstacle::TrailID
then Bumper's method will count the number of obstacles for a given trail record.
You can also define a summary field in Obstacle as the "count of" TrailID. If you then place this summary field from Obstacle on the Trail layout, it will display a count of the number of related obstacle records.