You can make hundreds of batches from a single sachet of yeast if you really care to. Harvest the slurry upon racking. Best practice is to "wash" the yeast...but I am lazy and sometimes just repitch a portion of the slurry into my next batch, You can also save the yeast cake in the refrigerator in a sanitized container...I prefer to just use ziplock bags as I figure they are sanitized during manufacturing, hopefully....hah. I try and reuse the yeast promptly and not let it "hang around" too long, how long is too long, IDK.
Best practice is probably not to split a pack of yeast, for the couple three dollars it costs.