Although it may seem like it's an arbitrary measure depending on how well you dip your fingers into the salt bowl a "pinch" is actually supposed to be a specific measure of 1/16th of a teaspoon. But because it is rather small, a tiny bit more or less usually won't be too much of an issue in the larger recipe. Usually.
It means what it says.
Take what you can pinch between finger and thumb - so not a lot.
Update
Historically the pinch was more precisely defined by some U.S. cookbooks as approximately 1⁄8 teaspoon.
In the early 2000s some companies began selling measuring spoons that defined (or redefined) a dash as 1⁄8 teaspoon, a pinch as 1⁄16 teaspoon, and a smidgen as 1⁄32 teaspoon. Based on these spoons, there are two pinches in a dash and two smidgens in a pinch.
One pinch of fine salt is approximately 1⁄4 gram (20-24 pinches per teaspoon), while one pinch of sugar is 1⁄3 - 1⁄2 gram.
a pinch is less than a tsp. it's just a very small amount
A pinch
That is about 14 teaspoons
At 6 tsp per ounce you get 10.14 tsp for 1.69 ounces. This is a smidge* over 10 & 1/8 tsp (10.125). *note: 1/8 tsp is considered the smallest amount you can measure, equal to a pinch. Smidge could be considered a dash.
There is approximately 0.3 teaspoons in 1.5 milliliters.
3 teaspoons equal 15 gm.
1 dash of vanilla extract is approximately equal to 1/8 teaspoon.
That is approximately 72 teaspoons.
756.8 teaspoons in a gallon.
That is 768 teaspoons.
That is approximately 3.5 teaspoons.
That is 44 teaspoons.