Are there more registered democrats or republicans?

This is a very good question. What makes it a very good question is the fact that it is very difficult to answer.

Thirty States in the USA require some kind of party affiliation (or non-affiliation) as a part of their voter registration system. So if you are looking for "registered Democrats or Republicans" you are limited to those 30 states and can't look in 20 states that do not require registration

If you are looking for the number of people who "consider" themselves as Democrat or Republican you can look to statistically significant surveys, such as the Rasmussen survey. Rasmussen reported 35.4% reported themselves as Republican and 32.7% reporting themselves as Democrats. 32.4% said they were "Independent." This polling asks 15,000 Americans to report their affiliation. This may seem like a few, but it is a very good number over all for determining the breakdown nationally.

You might ask 35.4% of what? What do I multiply 35.4% by to determine the number of persons who consider themselves Republicans. Too bad, we don't have that number, and whatever it is it changes every day.

There are several other major polls (Pew and Gallop, to name a few), which show slightly different numbers indicating a Democratic advantage, but, over time, the longer-term trend for a decade has been for more people to identify as Independent (now almost 40%) than either Democrat or Republican (both which vay from about 25 to 35%, depending on the year and poll). However, if the question includes "leaners" (self-identified Independents who usually vote for one party), the percentages change significantly. Democrats consistently have about 50%, while Republicans muster a variable 35-45%, looking at data from the past decade.

But in the end you must ask yourself exactly what is it that you want to know? Because when you include the peculiars of the many state parties and the peculiars of the country's election systems these numbers are frequently meaningless. In particular, the outcomes are driven by actual voters; the percentage of voters who actually vote varies significantly in any given election, with the percentage of registered voters who don't cast a vote varying from about 15% to as much as 30% or more.