If you are executing in one local machine, you can have only one robot to execute the task. However, if your desktop application is executed in multiple machines, then you should have one robot per machine to execute the task. So usually, when installing UiPath Studio, it installs the robot as well.
So, per machine, you will anyway have a robot. it all depends on where you want to execute
Then you can manage without the orchestrator. Orchestrator is required in cases where you need to manage the process centrally, in situation you need to schedule the process to run at a particular time and in cases where you need to paralell execute the process in multiple environments/ machines.
So, in case if you process is activated manually by a person, and you do not need such monitoring or scheduling, you can consider only the UiPath studio along with the robot. You also have the community edition of the UiPath studio which is free. You can decide whether it is good to go with the community edition as well…