Does keyword density in the image alt attribute effects the site's SEO?
Keyword density in the Alt attribute doesn't effect anything.
It does not affect, don't worry. If that was the keyword density in text, you have to reduct the amount of keywords.
"Keyword density" is a SEO concept of 1990's and maybe 2000's, but this does no apply any more. SEO evolve and this is one of the things that are now part of the past.
Keyword density only applied within any content for quality story but not on image alt.