How common is it for someone to need an anterior cervical corpectomy?

Individualized. This procedure is individualized by the surgeon's experience and your presenting disease. If the surgeon feels he cannot achieve the desired result by a 2 level diskectomy and fusion, he will perform a corpectomy. It is common for some surgeons to do many surgeries this way and others will do it only when necessary. If you are uncomfortable with this, seek a second opinion. Good luck.
Varies. There are many spine surgeons who perform correct omits, though the trend is less invasive, more target specific solutions, that will provide faster healing and more long term functionality. Seek high level expertise before committing and don't be shy about second opinions.