Hi Jenny--when we went 1:1 with 70 or so faculty laptops and around 300 student laptops, we added a help desk staffed by exactly one person. He was experienced and fabulous, but completely overwhelmed. the ratio for years one and two needs to be different from that for a mature program.
The truth is that now, with around 800 laptops, we only need 1.5 people to manage them all. (There are two full-time employees, but one of them has other responsibilities besides just help desk that account for 50% of his contract.) The population is just so much more self-sufficient, teachers are front line troubleshooters now, and students can help one another. This means that more of what makes it into the help desk is the "big" issues, but that makes the job more interesting for our in house geek squad! Also, we are making money as a Lenovo self-maintainer, which is well worth looking into.