number of large animals
Used for counting large animals such as cattle, horses, or elephants. The counter 頭 (とう) is specifically for large animals.
Count the number of cattle on the farm.
この動物園の象の頭数は三頭です。
The number of elephants in this zoo is three.
頭数 is a noun meaning 'the number of animals (or people)', while 頭 is the counter suffix attached directly to a numeral like 三頭.
Compound of 頭 (とう, counter for large animals) and 数 (すう, number).