About the meta-robots, how many values can I put?
<meta name="robots" content="noindex, follow, nocache, nosnippet, noodp, noydir">
Is this valid? Or multiple meta elements?
<meta name="robots" content="noindex, follow">
<meta name="robots" content="nocache, nosnippet">
<meta name="robots" content="noodp, noydir">
Is this possible?
What is the best practice?
As you mentioned in your question's first code block;
<meta name="robots" content="noindex, follow, nocache, nosnippet, noodp, noydir">
It is valid. You can combine them with commas use it in under 1 tag.
You can also find some good examples in this link