Like most PPC campaign systems, there is a default cap at how many keywords, ad groups and so on you can have per account. Microsoft adCenter has their limits to:
- Keywords: 10,000 per adGroup
- Ads: 20 per adGroup
- Keyword Account Limit: 100,000
So you can basically max out your keyword list on a per account level to 100,000 keywords. I suspect a phone call to Microsoft, if needed, can help increase that limit, but I am not 100% sure about that.
adCenterRep adds he is "I am happy to assist" those with questions at a WebmasterWorld thread.
Forum discussion at WebmasterWorld.