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So to answer your question. You can ad a whole whack of negative keywords. Thousands of them.
Google has limits for number of campaigns, number of ad groups within campaigns and number of keywords within ad groups. These are all set to a standard limit, which can then be increased if you ask your reps - there is meant to eb a limit of 2000 keywords within any ad group (including negative keywords)
Here is why, according to AdWords:
There is a 2,000 keywords limit per one ad group and 50,000 keywords limit for one account but you can have a few accounts if you really need them.
You can have up to 2,000 ad groups in one campaign and up to 25 campaigns in one AdWords account.
On the other hand, you can get it straight from the horse mouth here:
In which case you can't add anymore of those things until you delete something.
And where is the deleted stuff stored then?
In the past (three/four years ago), this could be changed based on the specific request. More keywords, more campaigns...
Not sure how it works today...