How much is a 1776 silver twenty dollar coin worth?

No such coin exists for a few reasons.

Number 1, all US coin denominations (that were intended for circulation) greater than $1 are in gold. A silver $20 coin would be huge, the size of 20 silver dollars. (On the other hand, a $20 gold piece is roughly the size of a silver dollar)

Number 2, there is no US coin struck with the date of 1776. Nor is there any other country using the currency of "dollar" that would have struck a $20 silver coin.

What you have is a fantasy piece, post a new question including what the coin really is, its weight, diameter, and all other information about it, because it is not a US coin. (and most likely isn't even silver)