First off, yes, it is possible to create dynamic captions, much in the same way you can use dynamic labels or tooltips (by dragging the fields you want into the Marks card and then selecting them from the "Insert" dropdown menu within the caption.
However, I'm not sure I understand exactly what you are trying to do. What is the expected outcome if a user selects 5 states?
To add to what Joe said, you can have all sorts of calcs do anything you want. Top-3 calcs, max, determine if you have only one selected ort more than one selected, etc. Whichever calcs you want to see displayed in the caption (or the title, if you do it that way) just have to be added to the data shelf. Once they're in the data shelf (or anywhere else on the sheet, of course) then the pull-down for inserting fields will have them in the list of available calcs to add. And you can format it all with nifty fonts and colors and sizes, etc. You can conditionally display calcs too. (Well, there isn't a conditional display feature, but you can set the calc's value to null under the condition(s) you don't want it to be displayed...)
What i am trying to create is if i select one data point it should display the insights of the data point in cation
if i select 2 or more data points it should talk about the insights like max value,min standard deviation and difference and average of the selected data points and count of the data points in captions.
Like wise if user selects any data points dynamically the insights of the chart display in cation.
@joe i have attached one sample chart what i did in power BI same as it possible in tableau.
When you say "select", is that through filters? Or are you saying, if the user clicks on a single one of the bars, then display one caption, but if he clicks on multiple ones, display a different one?
If selection is via a filter, then you can control all that within the sheet, and you can do it in the caption. But if you are talking about clicking one-or-many bars in the viz, I think you'll need to do that a different way. (And if so, I have ideas, but I don't want to flesh it out if I don't need to go that route.)