If you apply using only OTCAS, then they won't have to write more than one, unless for some reason the school wants an additional copy sent to them (which I doubt). Most people are fine with sending more than one copy, and as was previously mentioned, will just send roughly the same, if not the exact same letter to the schools to which you are applying. Some non-OTCAS schools do it online now too - you submit the reference's info in your online application, and they receive a link where they can drop in their letter. I can't think of any cases where the letters would go to you - it's standard for them to be submitted directly to the school, or to OTCAS. However, on OTCAS you are able to accept or decline the right to view the references, which you usually cannot do. (Though I personally think it is more professional if you do NOT accept the right to view them)
Just be sure that you provide all of the information to those whom you have asked to write a letter - which schools, the deadlines for each (for OTCAS, make it your deadline to submit your stuff, not the school deadline), and what they need to watch their inbox for, or where they need to send a letter.