The number of times the word thankfulness is mentioned in the Bible, depends on the version you look at.
For example, in the ESV and the ISV, the word occurs twice:
1Co 10:30 If I eat with thankfulness, why should I be denounced because of what I am thankful for?
Col 3:16 Let the word of the Messiah inhabit you richly with wisdom, teaching and admonishing one another with psalms, hymns, and spiritual songs, and singing to God with thankfulness in your hearts.
In the KJV it occurs once:
Act 24:3 We accept it always, and in all places, most noble Felix, with all thankfulness.
If you are meaning, “How many times is the idea of thankfulness, mentioned in the Bible?” that would be many, many more times. Thankfulness is vital to the children of God and is a theme laced throughout the Bible.
Col 3:15 Let the peace of the Messiah also rule in your hearts, to which you were called in one body, and be thankful.