Was Ruth Boaz's only wife

As far as we know from The Bible record yes. There is no record of him having a wife before. In the light of comments he made to Ruth, it appears that he never did get married before and that also he was considerably older than she, the implication being that before too many years she would be caring for an old man.

The kinsman of Elimelech, being requested by Boaz to marry Ruth, said, "I can not redeem; for I have a wife and have no right to take another in addition to her, lest she be a disturbance in my house and destroy my peace. Redeem thou; for thou hast no wife."