the Lalvin/lallemande sized home brew packs (5 grammes I believe) are designed for batches up to 5 gallons, so it's normal to use one pack.
Of course, it's all about the amount of cells needed and it's feasible to use one pack for larger batches or higher gravity batches if you make a big starter to inoculate the must with.
In the same way if you make a large starter, then pitch half of it and freeze or at least refrigerate the rest....
At the same time, you may also see recipes out there, that suggest using 2 packs, but I notice those are often ones with quite high level gravity readings for the must, so I'd guess it's about making sure that there's enough of a cell colony of yeast to ensure the fermentation runs Ok from the start (those type of recipes often suggest daily or twice daily aeration, staggered nutrient additions, controlled fermentation temps etc etc).