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Some websites I have seen in the past tend to have so many keywords in their meta tags that sometimes I feel that it can be too much. 

How many keywords do you tend to have in your meta tags and how do you work out which to use?

I tend to use keywords that are most relevant to my site and I tend to limit  myself to around 10 keywords that are most relevant and would be most popular.

I am like you I will stick to a few keywords that are relevant to my site and not go over board. Going over board with a lot of keywords can have an affect on you in the long run.

Words that are most commonly used on my sites are ones I go for or ones that are relevant to the genre of my site, it also makes it a lot easier to have a few keywords over a lot.