Your history book is... not very useful.
Let's correct some language first -
Either this concept is not clear in my head or this is worded bad but wouldn't there be no population sanction if Rhode Island had not approved the constitution in the first place?
First off, I think you're looking for popular sanction, not population sanction.
Second, 12 out of 13 states would definitely indicate popular sanction. (or, more precisely a majority of the voters of 12 out of 13 statues - or, since Pennsylvania cheated, a majority of the voters in 11 out of 13 states, plus enough of Philadelphia. (Pennsylvania never alerted non-English speaking voters of the vote; they pretty much just declared ratification without actually bothering to issue or count ballots).
Remember that in the end, Rhode Island refused to ratify the Constitution until the other states threatened to treat Rhode Island as a foreign power - which would have doomed Rhode Island. Rhode Island was pursuing a populist free money policy that was extreme by any standard.
So Rhode Island was irrelevant - your textbook overestimates the importance of Rhode Island. I don't think anyone's calculations included Rhode Island any more than I consider the vote of my pet turtle.
That said, the Constitution needed popular sanction - everyone needed to believe that the Constitution represented the government the people chose.
One of the things that doomed the Articles of Confederation was the requirement for unanimity; the 13 states couldn't have passed a resolution that there were two digits in 13. By the time of the ratification debate, if New York had advanced the notion that water was wet, the South would have voted that water was in fact both flammable and solid, but never wet. The Articles of Confederation had taught the States that the best strategy was to be the last vote cast and that it was wise to vote against your own interest if by doing so you could extract prices from the other States. I can't call the examples to mind, and I can't find my copy of the book, but Klarman offers some examples of strategic voting that were almost as silly as my example of "water is wet".
So the level of popular sanction was set at a majority of the voters in 2/3 of the states. (the Constitution was not submitted to the states, but to the voters, but that is outside the scope of your question.) A majority of the voters in 2/3 of the states was sufficient to ensure that everyone would accept the legitimacy of the resulting government - that's the way "popular sanction" should be interpreted.
Best references are Maier's Ratification and Klarman's Coup. The origin of the Constitution is complicated and fully of wonderfuly juicy politics, but it is difficult to distill down to fit within the requirements of a High School Textbook (which is usually the first introduction to a complex subject, but must be written to contain nothing complicated, objectionable or confusing. They are written to be fairy tales to pass the scrutiny of the Texas School Board, an institution respected more for it's ideological purity than academic standards).