It all depends upon what you consider the "IT" department. At the Church we have one major IT department called "ICS" which stands for "Information Communication Systems". This is where many of the IT folks work. This department is headed up by Joel Dehlin. It has various departments within it divided into "portfolio" groups, such as "Member Leader", "Family History", etc. ICS handles most of the technology work for the Church as an operational organization, such as MLS, lds.org, materials management, missionary applications, etc. It does not do any of the work for family history. That is done by a whole different organization. The Family History department has a large organization all focused on the genealogy applications (familysearch.org, paf, etc). Outside of these two large departments we have other large departments which will have technical people assigned as well. For example, curriculum has graphic artists, programmers, etc. that help with specific curriculum tasks. I can't give you a ball park number as a) I don't know it and b) that information has not been made available to the public. However hopefully you have an understanding how we are structured.
Tom