System Maximums and Limits
GAM can only support up to 800 but we allow clients to add up to 700 negative keywords as we reserve the other 100 for other target criteria. It's a limitation with Google and any other publisher on GAM would have the same limitations.
Please refer here: https://support.google.com/admanager/answer/1628457?hl=en
Keyword Matching
Please refer here: https://support.google.com/admanager/answer/177381?hl=en&ref_topic=1638397