Altbier - Q. WL Yeast packs?

Hi,

My names Vincent, I’m relatively new to brewing, I have only made up a few kit brews and now I’m looking at starting to BIAB.  I have most of the equipment I need, I’m just waiting for a spare weekend.

I want to make an Altbier before I start creating my own recipes, I have found a recipe, I have all the necessary ingredients malt/hops/yeast etc. 

Batch size: 19L,

Boil size: 24.5L

OG: 1.051

FG: 1.013

My question: Under yeast in the recipe it lists White Labs WLP029 x 2 liquid packs.  I have 1 liquid pack, from what I understand 1 WL liquid yeast vile is enough for around 19L? Is there a reason for using two? What are my options? Do I have to buy another one?

Any help appreciated. Excellent forum.

Cheers,

Vincent