How to find the number of Tags a post has?

for tags you can use 

$tags = get_tags();
$categories = get_categories();

$no_of_tags = count($tags);
$no_of_categories = count($categories);

refer this if you need more details :
http://codex.wordpress.org/Function_Reference/get_tags
http://codex.wordpress.org/Function_Reference/get_categories

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