I want to combine some adjacent parts in an assembly and reduce the number of parts.
Use the Combine feature commande. Follow the next simple example.
Hope I helped.
Answered with a tutorial: https://grabcad.com/tutorials/how-to-combine-some-parts-to-a-single-part-in-solidworks
you can use Join command in assembly
1- insert two component in assembly
2-mate these component in correct position
3- use join command