Explanation:

What is a bodhisattva?
The term 'bodhisattva' literally means 'one who has enlightenment as his/her essence', from bodhi (awakening or enlightenment) and sattva (essence). It is not simply another term for a Buddha, though: a bodhisattva is a being who is destined for enlightenment rather than one who has gained it already. A bodhisattva is also normally thought of as consciously working towards enlightenment: you can’t call someone a bodhisattva just because they might be enlightened in the future if they haven’t started making an effort yet. For this reason, the earliest use of the term 'bodhisattva' refers to Siddhartha Gautama before he gained enlightenment, and also in his previous lives.

In the Theravada, as in Early Buddhism, though, this is the only use of the term. There is only one Buddha per age, who is the trailblazer who discovers the Dharma. So for each age, at any rate, there is only one bodhisattva.
The bodhisattva vow
The mark of a bodhisattva in the Mahayana is that he/she has taken the bodhisattva vow. The bodhisattva vow is solemnly made before one’s master in a special ritual, and involves four pledges:

To save all beings from difficulties.
To destroy all evil passions.
To learn the truth and teach others.
To lead all beings to Buddhahood.