for a single Ad Group within a Campaign, how many keywords is too many? if they're mostly 3 to 4 words phrase matches and exact matches? is 200 too many? obviously, if they are so narrowly targeted, some of them will take a long time to build up impressions. does Adwords tolerate that? thanks
Difficult to say. Basically you want as many keywords as possible targeted to that particular ad's copy. 200 isn't too many *if* all of those keywords work well enough with the ad to get a decent CTR. If you have so many words that they can't all possibly fit what your ad says, then your CTR will suffer. CTR drops below 0.5% and google will choke your ads before they stop them running all together.
I have no problems with many keywords. Most of them will have very few impressions and almost no clicks but this does not contribute too much to the overall CTR. Blo.
I always start with a long list then after a fortnight I review the ad and break out those words that get high volume impressions and/or CTR. I give them their own even more targeted ad.
There's never too many- as long as the keywords are targeted and specific to the creatives (Ad text) to which they relate. I have AdGroups with several thousand keywords. In general, it is good to have alot of specific keyword variations because it helps shield your CTR for your more general, broad scope keywords.
Build up several campaigns simultaneously. Make one campaign use the long list of keywords and have the others use segments of the long list. As already said above, revise the keywords over a period of time and blitz any that aren't performing. You should be able to laser-target to get a higher ctr but also retain the option of ditching a campaign which isn't performing well enough.