as much as you can afford

is it unlimited?

Up to 2000 keywords per ad group, all match types. I believe that includes negatives, or at least used to. I also think it's only active keywords, paused and deleted keywords are not counted.

There's also a 2000 group limit per campaign and 25 campaign limit per account. These I know for sure that paused and deleted groups or campaigns don't count. If you make the math, you'd think you could have up to 100 million keywords per account but that's not the case. There is a 50,000 keyword hard limit per account regardless of the total number of campaigns and groups.

AdWords account limits

These are the limits for an AdWords account:
500 campaigns (includes active and paused campaigns)
20,000 ad groups per campaign
5,000 keywords per ad group
300 display ads per ad group (includes image ads)
50 text ads per ad group
3 million keywords per account
10,000 location targets (targeted and eAdWords account limits

These are the limits for an AdWords account:
500 campaigns (includes active and paused campaigns)
20,000 ad groups per campaign
5,000 keywords per ad group
300 display ads per ad group (includes image ads)
50 text ads per ad group
3 million keywords per account
10,000 location targets (targeted and excluded) per campaign, including up to 500 proximity targets per campaign*xcluded) per campaign, including up to 500 proximity targets per campaign*

Whoa Hakona, where you get these numbers? Unless Google made a change that I missed, these are all way above the numbers I'm familiar with except for the 50 ads per group one. Even if you ask to increase the limits, which they'll do at their discretion, the highest possible limits I'm aware of are 100 campaigns, 10 000 groups per campaign and 10 000 keywords per group and I believe one million keywords for the entire account.