The number of local workers available with Parallel Computing Toolbox and no MDCS has changed. When introduced, the limit was 4; this changed to 8 in R2009a; and to 12 in R2011b.
If you want to use 16 workers, you will need a 16-node MDCS licence, and you'll also need to set up some sort of scheduler to manage those. There are detailed instructions about how to do this here. Once you've done that, yes, you'll be able to do "matlabpool open 16".