The source is the book “American Locomotives, An Engineering History, 1830 -1880“ page 7 by John H White. It also says that a total of about 120 British locomotives were imported into the US between 1829-1841.
Ah yes, that book. Darn good reference for both understanding OTL, and (IMO) getting a sense of how an ATL might see steam locomotive evolution - there's a fair amount of engineering at work to develop them, even if the basics are worked out within a generation.
I wonder if there's an equivalent for other countries (and I wish there was - at least, Amazon doesn't mention one - one for the next era, while I'm engaging in hopeful thoughts).
So why did US lines embrace "standard" gauge? Even pre-ACW, it seems to have been in a strong position (if hardly the only one).
Given what I remember from White's book, sheer practicality was not the only consideration at work, but locomotive designers and railroad builders weren't stupid.