You can use as many eggs as you want to make a pound cake, it just depends on the size of the cake you want to make. If you want to make a traditional pound cake you would use 1 pound of each of the following ingredients: flour, sugar, butter, and eggs.
The number of eggs will depend on their size but a large egg typically weighs about 60 grams and since there are about 454 grams in a pound you would need 7 (420 grams) to 8 (480 grams) eggs. If you wanted to make a smaller cake, you would first weight the eggs and then adjust the amounts of flour, sugar, and butter accordingly.