I'm not to worry about how many or limitation. But if too much over keywords stuffing or over optimitation, Google will slap you. Try to think how's your wallpaper useful for your visitors.
Yes ! Irrelevant keywords are bad for Search engine and Too much of keywords usage ( 10+ % is best ) per wallpapers post have negative impact on search engine ,
Yes, there is specific keywords limitation in your blog. If you will add extra keywords in your blog then it will exceed keywords density in your blog. But there is option available there, you can vary or spun your keywords then you can use keywords as much as you want. Thanks
Use of the keywords: Keyword stuffing is very harmful of any types of the website. you should be use 60-70 characters in Title Meta Description: 156-170 characters Meta Keywords: 256 Characters (Google does'nt give preference to meta keywords), but other engines prefer meta ketwords.
Yes I agree with kalpanasharma. he told about the meta keywords in website oriented. But in blog, there is definitely a keyword limitation it is 200 characters Regards, Roger
You should use 2 to 3 keywords per 100 words and do not be spammer. Write unique and quality content with using of LSI.