If you are planning to just pitch the dry yeast onto the wort, 2 packets is advisable. Due to lower fermentation temperature, yeast reproduction to optimal population takes longer, and can result in ester/by-product formation if the initial population isn't high enough (you don't want this for a dry lager).
You can pitch 1 packet if you really can't be bothered, except it may require longer conditioning to get to the same quality. Other option is to make a starter, which is covered many times over in the forum database.
If you are relatively new to brewing and haven't done a lager, or a starter before; my advice is to pitch 2 packets of healthy yeast and concentrate on keeping your fermentation temperature and racking technique under control. There's nothing worse to having something small wrong with a beer, and not knowing where it came from because you tried x new things.
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