SEO question about meta tags?

Can I make meta tags like this for example: hamburger, hamburgers.
Basically make a singular and plural of every keyword?
Or can that be seen as search engine spam?

No not seen as spam, they are 2 different keywords.

Avoid adding too many meta keywords - max 3-5.

However, meta keywords have far less impact overall these days - google 'say' they do not take them into account at all anymore.