Learn English – Should I use “know” or “knows” in “How many of you {know/knows} English?”

What you have here is an ellipsis. The full sentence is:

From one gift come many [gifts].

Gifts is plural, and so is the verb. It's not the one gift that comes; it's many gifts that come from it.

Conversely, if it were "from one gift comes another [gift]", or "from one gift comes one more [gift]", or something like that, you'd use the singular.

Two general hints to help you recognize such things in the future:

1. many is a sure sign that you're looking at, or should be looking for, something plural
2. from is a sure sign that you're not looking at the nominative case, and so you're likely not looking at the subject of a clause (it's "he does", not "from him does", nor "from him do")

It's let the world know how you feel.

It's just because let is followed by the base form of the verb. (AKA infinitive without to.)

Let him know,
Let them know,
Let him/them knows.