As you can see in the attached screenshot, I’m unable to purchase any more NTO stocks. Why is that the case?
See this thread and directly - this answer
Thanks, but why is there a maximum number of shares that I can hold?
The decision was taken by the risk management team to be aligned with our risk management policy, @wdavis. You may find more information in the thread above. 
It’s answered in the thread - to some degree.
It’s worth noting that the max position size for the OTC listing, NTDOY, is much higher at 58,080, so you could buy that instead.