Do you mean convert a table to a string, or opposite?
To simple display a table number value, use this:
a={1,2,3}
nr=1
text=tostring(a[nr])
Edit:
I get it.
If the table is a global table, you can do this
ball1={1,2,3}
nr=1
thetable = _G["ball" .. nr][5]
_G is the global table.