Considering only policemen and not security personnel or other defense forces

Population of US: 300Mn

Area of US: 9.8 Mn km²

Considering police stations are allocated for a specific area. Considering one police station can cover 2000 km²

Giving total police stations: (9800/2)~5000

Each police station may have 200 policemen (Amin + Patrol + Other duties)

Thus total policemen: (200X5000) 1 Mn Policemen