To calculate a value with the Field Calculator will return a value for each record.
Xander, I believe Mark was planning on using the Calculate Value tool, not Calculate Field.
It should be mentioned for the good of the thread that Calculate Value is a very powerful tool that allows you to do a lot in Model Builder that would be difficult otherwise, by allowing you to a) execute a python snippet and b) return a single value cast to any data type needed to "pipe" into the parameter of another tool. Though for this use case you definitely get the cookie - Select Layer by Attribute or Make Feature Layer connected to Get Count is the best solution for sure.