I'm a fighter duelled to mage with 12 mage levels & I still only get two extra images (me & two fakes) when I cast mirror image.
While Tashia, a level 12 sorcerer is surrounded by images (7 or 8)
Is this something peculiar to fighter/mages or a bug?
Is there a simple test to see if my other spells are being cast at the correct level?

It's a bug. Mirror Image casts one image for every .5 caster level, but the game can miscalculate multi/dual levels, resulting in what you see.

I went back & checked, unmodded vanilla BG1 does it as well, it is just not as noticeable with lower level mages.
the number of images is linked to your fighter class in both games.
If you raise your fighter levels with shadowkeeper or gatekeeper you get more images.
I hope it is just for mirror image.

Using shadowkeeper to set my fighter level 1 lower than my mage level fixed the problem.
I'll just have to watch my thaco on the next level up.
I'm a grandmaster in longswords so I already have the extra attacks per round.