Generally, if you rehydrate a packet of dry yeast that's been properly stored, it will provide the needed cells for a 1.060 or lower OG 5-gallon ale. For a lager, double the amount of yeast.
If you're going to try liquid yeast, get used to making starters. The packs/vials may say "direct pitch", but they come out of the factory with just 100 billion cells and the count goes down quite a bit each month to where the shelf life (in the fridge) is only 4 months.