That's a question that made me want an answer. It seems that, as of 2016, it may have been a lot:
> Among the questions asked by pollsters was whether respondents believed that leaked emails stolen from Mrs. Clinton’s campaign manager and published by WikiLeaks prior to the Nov. 8 election “contained code words for pedophilia, human trafficking and satanic ritual abuse — what some people refer to as ‘Pizzagate.’ “
> ...
> Specifically, 9 percent of registered Republicans who responded to the question said the allegations were “definitely true,” coupled with 40 percent of Republicans casting the claim as “probably true.”[0]
A different December 2016 poll[1] also seems to put "yes"/"definitely" at it at 14%[1] with a lot of "maybes".
Given the margins in US Presidential elections, it seems like the answer is "enough to matter". (Not that I think PizzaGate turned anyone from voting for Clinton or to voting for Trump, but fake news stories probably swayed a lot of voters overall.)
[0] https://www.washingtontimes.com/news/2016/dec/28/pizzagate-t...
[1] https://www.publicpolicypolling.com/wp-content/uploads/2017/...