Personally I'd go for the one without the dash - but if both are available, snag 'em!.

I presume by your second question you mean how many words maximum should you use in a domain?.

If that is what you're asking, I always try and aim for no more than three as something like freeshittynappies.com looks much better than getyourfreeshittynappieshere.com...

I agree that a 3 keyword site looks better. But if a long tail keyword of 5 or 6 has 1000+ daily searches on google, it would be a good idea so that I can rank for that keyword easily.

OR.

Is there some limitation in google so that it won't help me? (say it considers only up to 4 keywords, or something like that).

HH..

Dash it up, unless this is a site that you would consider to be more viral than SERP oriented. If people have to remember the site name and could spread it through word of mouth, go without dashes...

If it's for seo only it doesnt matter, if your trying to brand the domain, def no dashes...

Really does not matters from an seo perspective, But can be difficult for branding the site and making your users remember the site, as it really sucks to type in those dashes...

It depends on your goal, to brand or for shear seo purposes.

Dashes vs. underscores..