I don't expect it to out-sell Xanathar's Guide, but maybe Volo's.
And what has been going on with the SCAG? It always seems to get left off these sales-rank lists, even though it was the first 5E supplement and is therefore a good benchmark for comparison.
There is quite a bit good n the book, particularly if one likes the Forgotten Realms. But, the WotC folks have more or less admitted that it underperformed to expectation, and wasn't received well. Hence why we have a Volo's follow-up now, but no SCAG like follow-up.SCAG is #1,189 right now on Amazon. Its #12 on in fantasy gaming, just below the starter set and above all the adventures.
I don't think it opened as strong as say Volo's, but can go through some old threads to check. I don't remember liking it very much, seemed a bit muddled.
There is quite a bit good n the book, particularly if one likes the Forgotten Realms. But, the WotC folks have more or less admitted that it underperformed to expectation, and wasn't received well. Hence why we have a Volo's follow-up now, but no SCAG like follow-up.