It's a good idea to send an acknowledgment to each person who applies for a job. Doing so will save you from getting calls from applicants wondering whether you received their applications, and will keep you in good graces with prospective employees.
It's fine to keep acknowledgement letters brief and to the point. You may simply write something like:
We recently received your application for [job title here]. We will be reviewing all applications and making a decision in [number or date here] weeks. We will contact you at that time if an interview will be scheduled. Thank you for your interest in [your company name here].
Sincerely, [your name here]
If your applications are being received via email, you can send an electronic acknowledgement letter, using the same text. You can save it as a form letter within your email program, so you can send to all recipients by changing only the heading for each.