Do one more curl.
I can't, my left arm gives out with one rep less; unless I assist my left arm up and focus on the negative on the last rep of each set.
Do the same numbers of reps with both arms. One less on the right arm if you have to. The left arm will catch up quickly.
this is pretty common if you're more dominant with your right hand. Is there a visible size difference?
Every time I do curls, my left arm always gets one less curl than my right. What's the best way to fix this?
Ditch single arm specific stuff. Stick to movements where both are actively involved like Barbell curls, Ez bar skull crushers etc.