The literal Hebrew is, obviously, the spirit, which is found in a few translations:
“Then the Spirit stepped forward, stood in front of the LORD, and said, ‘I will deceive him.’ “ ‘How?’ the LORD asked. (GW)
Finally, a spirit came forward and stood in front of the Lord. The spirit said, ‘I’ll get Ahab to do it.’ (NIRV)
'And the spirit goeth out, and standeth before Jehovah, and saith, I -- I do entice him; and Jehovah saith unto him, By what? (YLT)
ויצא הרוח ויעמד לפני יהוה ויאמר אני אפתנו
The are several reason why most English readings are simply "spirit."
Since English has both definite and indefinite articles, a passage with "the" spirit, is typically understood as "the Spirit" not "the spirit." The definite article typically is understood to mean the Spirit not simply a singular spirit. Even if "the" spirit was not the Spirit of God or the Holy Spirit, as is the case in this passage, "the" in English is not understood as simply as an ordinary spirit. [In English that would be conveyed using the indefinite article, "a" spirit.]
The NJPS does a good job of expressing the idea in the original text:
until a certain spirit came forward and stood before the LORD and said, 'I will entice him.' 'How?' the LORD asked him.
What is stated is a singular spirit came forth. One could say, it was definitely only one spirit, but the writer omitted further identification.
Additionally, First and Second Kings often uses an article in a manner which is different from English usage. The best example of this is seen in the repeated use of הרע to describe the actions of someone:
and Solomon doth the evil thing in the eyes of Jehovah, and hath not been fully after Jehovah, like David his father. (1 Kings 11:6 YLT)
ויעש שלמה הרע בעיני יהוה ולא מלא אחרי יהוה כדוד אביו
Even the YLT struggles to convey the literal literal Hebrew: the evil,doesn't make sense. The English requires an ellipsis, thing. In English, "the" evil means something singular, yet "the evil" done by Soloman is plural. Again, the NJPS does a good job of expressing the passage
Solomon did what was displeasing to the LORD and did not remain loyal to the LORD like his father David. (1 Kings 11:6 NJPS)
הרע is found 93 times in the OT, 10 of which are in First Kings, and 22 in Second Kings. In each case there is no singular "evil" the person did; rather the use of the article functions as a means of giving emphasis. It should not be understood as intended as a numerical count, even in cases where the meaning is singular.