It also depends on how much wort you're brewing, but I for example always brew half batches only, e.g. 10L instead of 20L, and I always use all of the yeast in my half batch, as if I was fermenting a full batch, and all my batches turned out great (except my latest lager experiment,but that's a different story). There is however an issue if you seriously underpitch or overpitch, just google it to get more insight, I am not an expert on how many billion yeast cells are required for all the different styles and amounts of beer, and hence don't want to give you wrong info...