How to interact with a Numbers sheet via AppleScript?
As far as I know, Numbers does not have an explicit AppleScript command to select a given sheet.

There is the active sheet property of the document, e.g.,

tell application "Numbers" to get active sheet of front document


which will return the active sheet; however, while one might think that e.g.,

tell application "Numbers" to set active sheet of front document to sheet 2


assuming there is a sheet 2, would set e.g., sheet 2 to the active sheet, it does not set it as the active sheet. 

Here is the method I use the makes a given sheet the active sheet in the front document:

Note: This is a hack method and it does cause a saved document to show as edited, even though it's not altering the value of the target cell. It works by setting the value of cell "A1" to it's current value, so while it's perceived as an edit, no data has been altered from its current value. However, it does work for me in Numbers 5.1 in macOS High Sierra, and is why I use it.

my selectSheet(2)

on selectSheet(n)
    try
        tell application "Numbers"
            activate
            set value of first cell of first table of sheet n of front document to ¬
                (value of first cell of first table of sheet n of front document)
        end tell
        --  # Press escape key to remove focus from cell A1.
        tell application "System Events" to key code 53
    on error eStr number eNum
        display dialog eStr &amp; " number " &amp; eNum buttons {"OK"} ¬
            default button 1 with icon caution
        return
    end try
end selectSheet




Note: The example AppleScript code is just that and, sans any included error handling, it does not contain any other error handling as may be appropriate. The onus is upon the user to add any error handling as may be appropriate, needed or wanted. Have a look at the try statement and error statement in the AppleScript Language Guide. See also, Working with Errors.