Dumbbells don't spawn, the are found in specific locations.

1. Public Gym lower left corner.
2. Police Gym, lower right corner.
3. Rays' Farm House 2nd floor right side
4. Heather's Home front yard, by garage (event triggered)
5. Public Gym 2nd floor locker room, third locker in the center section, key required,
6. XXX Shop in locked chest, back right corner.
7. Ray's Farm House, 2nd floor, Hanna's Room, lower right corner.
8. Warehouse, upper left corner. (Requires story advancement).
9. Ella's apartment, metal jar in wall of camera room.
10. Heather's home, Heather's bathroom, lower left corner.

how do you trigger the garage one and locker ?

The one by the garage is part of the peeping tom part of the story. The one by the locker requires you to get a locker key, but I don't know which one it is. There are two in the Public Gym, on the climbing wall lights, but you need the grippy shoes to get them.

so it did have 1 extra dumbells