The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus and adrenal glands. The hypothalamus and pituitary gland are neuroendocrine organs.
A newspaper lead (or lede if you prefer) can do its job in one sentence, but with few exceptions, a paragraph will contain more than one sentence. The OWL site, aimed at college students, suggests a length of from three to five or more sentences.
Obviously, the longer your essay, the greater the number of paragraphs it will contain. As others have suggested, a very rough average for a paragraph is 100 words. So, a 500-word essay would, by this reckoning, have approximately five paragraphs. Much also depends on your topic.