What is the number of - COOH groups in a citric acid molecule?

Citric acid is a tricarboxylic acid with the IUPAC name $2$-hydroxypropane-$1,2,3$-tricarboxylic acid.

The chemical structure of citric acid is:

$\text{HOOC}-\text{CH}_2-\text{C}(\text{OH})(\text{COOH})-\text{CH}_2-\text{COOH}$

From the structure, it is clear that there are $3$ carboxylic acid ($-\text{COOH}$) groups present in a citric acid molecule.

Answer: $3$