Are you on iOS or Android?
On iOS, tap the Sends tab in the Effects screen, then tap the Green buttons to the left of each send effect to toggle it for this track.
This is what the screen looks like:
On Android, send effects are not yet available. There is a workaround, but it’s quite limited:
To have for example the Delay effect affect tracks 1 and 3 only, enable the “Limit to FX tracks” checkbox in the delay effect. In the track list, enable the blue FX button for tracks 1 and 3.