Because of the current seiyuu boom (late 1990's, there are MANY seiyuu related events. Some of the events are tied into anime or games, some of the events are for promoting the seiyuu's CDs or books, and some of them are just talk events by the seiyuu. There are also a growing number of seiyuu who perform concerts, and there are some seiyuu who are stage actors and actresses.
The exact number of seiyuu related events is unknown, but
I would estimate that there are over 500 every year.
Although the seiyuu boom seems to be weakening, the number of seiyuu events seem to be increasing. There is also a bigger cross over between the minor idols and seiyuu.
However, instead of the large events for thousands of people,
there are more and more smaller events for one hundred or so people.
So I think there may even be more than 1000 events a year.
Since 2015 or so, more and more anime and game related events
are being broadcast live on the Internet.
Some event reports (by me and my friends) can be seen below.
search the event database
The search string can be anything, like a seiyuu name, show name, etc..
Some regular expressions will work, and all searches will be done in a
case insensitve manner.
When typing in names, use the "LASTNAME FIRSTNAME" order.