There are 20 million accounts, 125 thousand active accounts [1]. The number of people who refer to Wikipedia is probably over a billion, since it's often the first hit in Google (speculation). Roughly 25% of edits are anonymous (no account), and anonymous edits are significantly longer than non-anonymous ones [2].

[1] https://en.wikipedia.org/wiki/Wikipedia:Statistics

[2] https://meta.wikimedia.org/wiki/Research:Anonymous_edits