What do the numbers in
Circuit handshake stats since last time: 1833867/1833868 TAP, 159257/159257 NTor.
mean? Is it how many of them were completed?
Yes, that's correct.
There are two types of circuit handshakes -- the original, called the Tor Authentication Protocol in Ian's paper about it: http://freehaven.net/anonbib/#tap:pet2006 and then the better one, called NTor: https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/216-ntor-handshake.txt https://gitweb.torproject.org/tor.git/blob/refs/tags/tor-0.2.4.17-rc:/ChangeLog#l769
You can read about the switch to NTor, and the reason we added the log message you're asking about, in this blog post: https://blog.torproject.org/blog/how-to-handle-millions-new-tor-clients
So yes, the fractions x/y mean that y requests of that circuit handshake type came in, and your relay completed x of them. The rest were refused because your CPU couldn't keep up (by refused I mean that your relay sent back a destroy cell, to tell the client that it isn't going to happen).
On high-bandwidth relays these days, I wouldn't be surprised to learn that many of the TAP handshake requests are still refused. Many of the bots have been cleaned up, but many of them still remain too.