A half pack will almost certainly be enough. You won't do any harm at all if you want to use the whole pack. Even if overpitching is a thing at the home-brew scale (I am not convinced), a factor of 2 is fine.
Too much is better than not enough. But I'd say you probably need at least 6 grams of that 11 gram packet.
So a little more than half. But if you don't like the idea of sealing your sachet up and using it later on for another brew, then just pitch the whole thing. It won't hurt anything.
edit.... just ran the pitch rate calculater @schmurf linked. Seems I'm wrong and you only need 4.9 grams. But still, more is usually better than not enough. On typical homebrew scales you aren't going to experience the bad things you hear/read about from overpitching. Unless perhaps you just grossly overpitch by some ridiculous amount. However most of us probably would find that too uneconomical. Even for dry yeast.
2-3 grams is plenty. 1/2 pack is more than plenty. 1 whole pack would be a waste. Use a partial pack and save the rest in your refrigerator to use for a future batch or four. As a small batch brewer I do this all the time, many many years.
hi all. thanks i got too lazy to vacuum seal the leftover yeast and just dumped the whole pack. i pitched at 68. and within 20 mins i got a thin foam already forming. the notty took off like a champ .