Taxonomy term count per vocabulary
For count the number of term in a vocabulary you can use this method : taxonomy_get_tree($vocabulary_id). It will returns all the terms of the vocabulary in an array. After that, you only have to get the size of the array.

Example :

$terms_array = taxonomy_get_tree(9); //9 is the vocabulary ID.
$terms_nb = sizeof($terms_array); // $terms_nb = your number of terms.