Within the context of your question, I take dispensationalism to mean those Christian denominations who believe the Church has NOT replaced Israel in God’s program and that the Old Testament promises to Israel have not been transferred to the Church, that God is not finished with Israel and that Israel will feature prominently in the tribulation period before the millennial reign of Christ is established. It is my understanding that Catholics, Anglicans, Latter Day Saints and Jehovah’s Witnesses believe they have replaced Israel (spiritually speaking).
The United States has the largest Christian population in the world, with nearly 240 million Christians. Source: https://en.wikipedia.org/wiki/Christianity_in_the_United_States
From the total number of Christians in the U.S. in 2017 240 million
Subtract all the Catholics 70.4 m
Subtract all Latter Day Saints (Mormons) 6.6 m
Subtract all Jehovah’s Witnesses (neither Protestant nor Catholic) 2.5 m
Remainder of Christians in the U.S. 160.5 m
That leaves an approximate number representing non-denominational Bible churches, some Baptists, Pentecostals, and Charismatic groups who subscribe to dispensationalism. Not sure where Anglicans or Methodists stand on dispensationalism and Israel, but they come under Protestant.
I suspect that the total percentage of Christians in the U.S.A. who uphold the dispensationalist view that the Church has NOT replaced Israel in God’s program and that the Old Testament promises to Israel have not been transferred to the Church is not as high as some might suspect, given the lack of statistical information about non-denominational, evangelical and charismatic groups.
Disclaimer: I am a complete dunce when it comes to maths, ending up in the bottom of my class at school. Also, I know nothing about demographics or how statistics are arrived at, but here are my sources:
Source of statistics for U.S.A. in 2015: https://en.wikipedia.org/wiki/Christianity_by_country
Catholics in the U.S. 70,412,021 (22% of population) (2017)
Jehovah’s Witnesses in the U.S. 2.5 million (0.8% of population) (2017)
Latter Day Saints in the U.S. 6.6 million (9.2 m North America, 16.1 m worldwide)
EDIT: I am not trying to come up with "an accurate number" because that would be utterly impossible given the confusion within various sources of demographic information available. Below is a Wiki link for the religious demographics of the United States. Make of it what you will, bearing in mind that there are zero statistics to say how many Protestant denominations support Israel: