There are two schools of thought on lager fermentation. You can pitch warm to get the yeast started and then drop to 50-55º but you will probably need a diacetyl rest. Starting the yeast at the higher temperature means that you can get away with using less yeast but it will produce esters and other flavor compounds as it reproduces. Those will need to be cleaned up with a diacetyl rest and slightly longer lagering. You could probably get away with one vial or smack pack but two vials or a 2L starter would be better.
Or you can pitch slightly below fermentation temperature (43-48º) and let the wort rise to your target 50-55º. This will give you a cleaner fermentation with less chance of off flavors and will probably eliminate the need for a diacetyl rest. This approach, however, requires a larger pitch of yeast.
The general rule of thumb is 1 million cells per milliliter of wort per degree Plato. That is 1 million times 19000 (19 liters in a 5 gallon batch) times 12, 13, or 14 for a 1.048, 1.052, or 1.056 lager respectively (one degree Plato is about 4 specific gravity points). That puts you in the ballpark of 2.5 to 3 billion cells, which works out to 3 vials or smack packs to be on the safe side.
If, as some have suggested here, you make a starter as large as 1 gallon, you'll want to make it about a week ahead of time. That way you can allow the starter to ferment all the way out, put it in the fridge to settle, then decant the spent beer, leaving only enough to swirl up the yeast for pitching. Otherwise you are diluting your carefully crafted wort with a gallon of unhopped beer that was meant to grow yeast rather than taste good.
Jamil Zainasheff's Starter FAQ
has a lot of good information about pitching rates and starters.