10,000 Jedi (Knights) around the time of Star Wars Episode I: The Phantom Menace according to the sources I detail below.
First, according to the Star Wars Episode I: The Phantom Menace novelization by Terry Brooks and published on April 21, 1999:
In the time of Qui-Gon Jinn, ten thousand Jedi Knights in service to
the Republic carried on the struggle each day of their lives in a
hundred thousand different worlds spread across a galaxy so vast it
could barely be comprehended.
Source: an Amazon.com Look Inside search for "ten thousand" on its page for the novelization.
Second, according to the young adult book Episode I Adventures 1: Search for the Lost Jedi published September 1, 1999:
... twelve member Jedi Council contemplated the Force and decided the action of over ten thousand Jedi Knights throughout the galaxy.
Source: a Google book search for "ten thousand Jedi".
Third, according to a description on the LucasArts website for the video game Star Wars Episode I: Jedi Power Battles released March 31, 2000 in North America:
Out of 10,000 Jedi in the Galactic Republic, you are the one.
Chosen by your peers, you now stand in the Jedi Council Chamber high
above the bustling metropolis of Coruscant. Encircled by great Jedi
who sit in the stately chamber, a mission of great importance to the
Republic is about to be bestowed upon you.
Source: Archive.org Wayback capture of the LucasArts website dated June 15, 2000.
Aside: If we had access to GFFA census data like we do for many English speaking countries, this question would be a lot easier to answer. ;)