For the cost of a wine yeast you really want to use a whole pack. The cost of not using the whole pack is that if your hydration process is not good you might kill more than 50% of the colony AND unless you know that any bacteria that have access to the open pack of yeast are not going to be thriving on the dehydrated yeast cells that have been packed with a tiny amount of nutrient then you are asking for trouble storing an opened pack of yeast in your fridge. Supplies for your wine or mead making by magnitudes outweigh the cost of a packet of yeast... so to be penny wise, don't be pound foolish. One pack of yeast can ferment up to about 5 or 6 gallons of a low ABV wine.. Home wine makers cannot possibly over-pitch yeast, but we can - and do - frequently under-pitch.