How many total speakers of German were there in pre-WWI USA?
The 1910 US Census enumerates some 2.7 million foreign-born speakers of German in the United States then. That number must be an understatement because there were many communities of heritage German speakers in both rural and urban areas of the US at the time. Do we have any idea how large this language community was in the United States at the time?
The census switches over from foreign born speakers to both native and foreign in 1980. The result is a 25% increase of German speakers counted assuming no decline in foreign German speaking population between 1970 and 1980. It is likely that there was a decline between then meaning that even in 1980, the ratio of native to foreign speakers of German was over 25%. In the past it may have been even higher before those communities were subjected to language assimilation during the period from WWI to WWII.
I do not know for a fact how much German immigration there was to the US in the early 1900s and in what places, but I am led to believe that German immigration to Texas had largely ceased by 1910. However, a community of around or possibly over 100,000 native German speakers still existed in the state. These people would not have been included in the above 2.7 million foreign born German speakers. (Note that the estimate of Texas German speakers was simply a commonly quoted number I saw in several news pieces via google; I don't know how reliable it is, though the modern population is around 8,000).