That is a higher number than supported impeaching Nixon before the Watergate hearings. 

Courtesy of Gallup:

Gallup’s preceding poll, conducted June 3-16, put Trump’s job approval rating at 43%. In the same survey, the slight majority of Americans were opposed to Trump’s being impeached and removed from office over the findings of the Mueller investigation — something an increasing chorus of Democratic Party leaders have called for.

When asked for their opinion “based on what you know about Robert Mueller’s investigation into Donald Trump’s activities,” 45% of U.S. adults said Trump should be impeached and removed from office over the matter, while 53% said he should not be.

Republicans show similar attitudes on the two dimensions, while there is more separation for Democrats and independents.

Eighty-nine percent of Republicans in the early June poll said they approved of the job Trump is doing, and 92% thought he should not be impeached.

By contrast, 6% of Democrats approved of Trump and 18% said he should not be impeached.

Only 37% of independents approved, while 51% said no to impeachment.

It should be noted that this number is higher than for both Clinton and Nixon in the early days. (Actually for Clinton it never got that high.): 

For the entirety of Gallup’s trend on Clinton, public support for impeaching him never exceeded 35% (that was recorded in mid-September 1998), but it was as low as 19% in the initial reading in June 1998.

The range of support for impeaching Nixon in the early months of the Watergate investigation that led to Nixon’s downfall was similar to Clinton’s. From June 1973 to January 1974, between 19% and 38% of Americans agreed he should be “impeached and compelled to leave the presidency.” However, in April 1974, using a more complex question wording that defined the impeachment and removal processes, Gallup found support for removing Nixon at 46%. And by the final reading in late July — one week before Nixon resigned from office — support for removal reached majority level for the first time, at 58%.

My feeling is that this number is going to spike right after people see Robert Mueller testifying. 

And after that, I think Congress will have no choice but to move forward.