how many black 10’s are in a deck of 52 cards?

In a standard deck there are four suits. Two red and two black. Hearts and Diamonds are the red, Spades and Clubs the black. Each suit contains 13 cards, one each of Ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen, and King. Since you have two black suits, you will have two black tens.
Red And Black 10s
It depends on your frame of reference, and the curvature of the universe:

At sufficiently high enough speeds ~c, the two seperate events of playing two black tens may coincide into one event, which reduces the numbers in the pack.

Also, in a perfectly curved universe, an observer reading his own hand, and then looking into the ‘distance’ would see their own hand and their own hand again as the light curves around the surface of the earth (assuming that the earth’s g is strong enough to orbit light) and thus the observer would see double the amount of black ten’s at any one time. This number may treble, quadruple, etc depending on how far the observer sees.

OR

There are 2 black tens in a standard 52 deck of cards. Count’em.
real deck 2

black jack standard 8