Perfect squares?

Number Theory Level 4

Let $N={ 1 }^{ 9 }\times { 2 }^{ 8 }\times { 3 }^{ 7 }\times { 4 }^{ 6 }\times { 5 }^{ 5 }\times { 6 }^{ 4 }\times { 7 }^{ 3 }\times { 8 }^{ 2 }\times { 9 }^{ 1 }$ . How many perfect squares divide $N$ ?

Perfect squares?

Source: old IWYMIC problem.