There are grounds for believing that "home-grown" right wing terrorists have caused more incidents than Islamic terrorists (but the number of deaths is similar)
The question has been addressed by the non-partisan Government accountability Office.
Their report(pdf) argues the following (my highlights):
White supremacists, anti-government extremists, radical Islamist extremists, and other ideologically inspired domestic violent extremists have been active in the United States for decades. Examples of attacks include the 1993 World Trade Center bombing by radical Islamists, in which 6 persons were killed; and the 1995 Oklahoma City bombing of the Alfred P. Murrah federal building by anti-government far right individuals, in which 168 lives were lost. The September 11, 2001, attacks account for the largest number of fatalities in the United States in a single or closely- related attack resulting from violent extremism in recent decades. While the September 11, 2001, attacks were perpetrated by foreign violent extremists, from September 12, 2001 through December 31, 2016, attacks by domestic or “homegrown” violent extremists in the United States resulted in 225 fatalities, according to the ECDB. Of these, 106 were killed by far right violent extremists in 62 separate incidents, and 119 were victims of radical Islamist violent extremists in 23 separate incidents.
If this count is credible, it appears that whether you count incidents the "home-grown" right wingers have caused more terrorism in the USA than Islamic extremist inspired terrorists (though the number of deaths is similar).
They do, however, caveat this claim with the following:
Since September 12, 2001, the number of fatalities caused by domestic violent extremists has ranged from 1 to 49 in a given year. As shown in figure 2, fatalities resulting from attacks by far right wing violet extremists have exceeded those caused by radical Islamist violent extremists in 10 of the 15 years, and were the same in 3 of the years since September 12, 2001. Of the 85 violent extremist incidents that resulted in death since September 12, 2001, far right wing violent extremist groups were responsible for 62 (73 percent) while radical Islamist violent extremists were responsible for 23 (27 percent). The total number of fatalities is about the same for far right wing violent extremists and radical Islamist violent extremists over the approximately 15-year period (106 and 119, respectively). However, 41 percent of the deaths attributable to radical Islamist violent extremists occurred in a single event—an attack at an Orlando, Florida night club in 2016 (see fig. 2).
The caveat at the end of the quote reflects the difficulty of using simple summary statistics for rare events as any average is highly skewed by single large events.
It is also worth noting that Islamic attacks get a lot more news coverage than others according to this study:
Controlling for target type, fatalities, and being arrested, attacks by Muslim perpetrators received, on average, 449% more coverage than other attacks.
A news story related to the incident in Charlottesville reports that the FBI and Homeland Security have assessed the risk of attacks over the last couple of decades by extremist groups and concluded that:
...white supremacist groups had already carried out more attacks than any other domestic extremist group over the past 16 years and were likely to carry out more attacks over the next year...
The source document which appears to have emerged from Homeland Security seems to clarify some of the previous claims and provide additional estimates of the future threat as well as the historic patterns.
This seems to clarify and reinforce the earlier claims in this answer.