Essential prime implicants are the prime implicants that cover at least one minterm that no other prime implicant covers. To find the number of essential prime implicants for the given function Y = ABCD ABCD ABCD, we need to first determine the prime implicants.

Prime Implicants:
A prime implicant is a product term that covers at least one minterm of the function and cannot be combined with any other prime implicants to cover more minterms.

To find the prime implicants, we can use the Quine-McCluskey method. Here is the step-by-step process:

1. Write down the minterms in binary representation for the given function. In this case, the function Y = ABCD ABCD ABCD is already in minterm form, so we can skip this step.

2. Group the minterms based on the number of 1's in their binary representation. Start with grouping the minterms with the fewest number of 1's.

Group 0: None
Group 1: ABCD ABCD ABCD (All minterms)

3. Compare each pair of minterms from adjacent groups. If they differ by only one bit, create a new group with the combined terms.

Group 0: None
Group 1: ABCD ABCD ABCD (All minterms)
Group 2: None

4. Repeat step 3 until no new groups can be formed.

Group 0: None
Group 1: ABCD ABCD ABCD (All minterms)
Group 2: None

5. Identify the prime implicants by selecting one minterm from each group. In this case, the prime implicants are ABCD ABCD ABCD.

Essential Prime Implicants:
To determine the essential prime implicants, we need to find the minterms covered by each prime implicant and check if any minterm is covered only by a single prime implicant.

In this case, all the minterms are covered by the prime implicant ABCD ABCD ABCD. Therefore, this prime implicant is the only essential prime implicant.

Conclusion:
The number of essential prime implicants for the function Y = ABCD ABCD ABCD is 1.