Some teeth have only one root, while other teeth normally have 2, 3, or more roots. It depends on which teeth we are talking about.
Incisors (front teeth), canines (pointed teeth next to incisors), and 2nd bicuspids (small teeth right in front of the large molars) typically have a single root.
Lower molars and upper 1st bicuspids typically have two roots.
Upper molars usually have three roots.
There are insignificant variations, called anomalies, where a tooth can have more or less roots than what is considered normal.