Roughly (but the upper end especially is quite variable and idiolectic): a couple of people: 2 to 3 dozens of: 20 to 100 hundreds: 200 to 2000 thousands: 2,000 to 50,000? tens of thousands: 20,000 to 120,000 hundreds of thousands: 200,000 to 1,000,000 millions : 1,000,000 to 90,000,000? -- It is thousands when no number precedes ('I have thousands of enemies') and thousand when a number precedes ('I have ninteen thousand enemies').
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.