LEAST POSSIBLE MINIMUM: 13 Jedi
-13 Jedi is the absolute minimum and its almost 100% sure that it were more as it assumes that all his lightsabers he used in the clone wars 2008 series were from the dead jedi he also collected VISIBLE trophies from in episode 1x10 which were 12 plus the kill of Nahdar Vebb.
Possible Maximum: 13-21+ Jedi
-Assuming that there were no or only partialy as much as lightsabers in his collection as strands of hair from dead Jedi and he had to replenish them by new kills. We can atleast assume more then 13 kills as parts of the showcases of his strands of hair collection werent visible to the angle or clone troopers staying in front of it. But they were reffered by Jedi Knight Nahdar Vebb as "SO MANY!".
Here is where I draw those information from:
I just counted all the lightsabers from his collection we were able to see to estimate the amount of Jedi he killed in canon.(So the amount of lightsabers or other trophies we saw in ROTS and The Clone Wars 2008)
In the clone wars series Grievous lost several times a saber or was disarmed for example he lost 4 lightsabers alone at his short time capture by the EMP wielding Gungan Grand Army.
He lost 8 lightsabers during the whole Clone Wars 2008 Series.
SO if we count every lightsaber he lost during the series and had to replace plus the trophies we see in his secret hideout in TCW 1x10 we can conclude the canon numbers. By doing this I was able to make an estimate on his canon kill count.
There are 12 visible trophies of murdered Jedi in his secret hideout.
VISIBLE and therefore again a MINIMUM!
As we arent able to see how big the room is and how many showcases were in it. Also some Clone Troopers partially hindered sight on the showcases.
MINIMUM: 49 Jedi (and some younglings)
MAXIMUM: HUNDREDS as stated by a Jedi in front of the Jedi Council in one of the comics.