Commercial packs of wine yeasts for home users generally are sized for a 5 gallon batch (usually anywhere from 1 to 2 grams per gallon of active dry yeast), designed for use in an average starting specific gravity. If you happen to use a full packet of D47 for a single gallon, that won't hurt a thing (all it means is that fewer generations of yeast cells will have to occur to build up to a stable population), but if you want to go appreciably larger than 5 gallons, or if you're starting from an initial gravity much higher than that typical of wine musts (say anything over 1.115 or so), it would be a good idea to pitch two packs.
You can pitch too much yeast, but generally that involves LOTS of yeast (say 10 to 20 packets into a 5 gallon batch). Likewise, if you pitch too little yeast then the cells need to undergo reproduction for more cycles to build up a stable colony, which could introduce mutations in the population, or if there isn't enough nutrient or oxygen to sustain that reproduction, you'll be left with too few cells to do a good job and the fermentation might stick at a higher than desired gravity.
