A good cover letter should be short, but if you apply for many different jobs positions, writing a unique cover letter for each one will overwhelm you. This might tempt you to write a single generic letter, change only the company name, and send it out for every job opening that matches your profession.
Each corporate job opening averagely attracts 250 resumes and of these candidates, only 4 to 6 are called for an interview. 98% of resumes and cover letters are rejected as irrelevant.
Never use the same cover letter for different job applications.
Instead of sending generic resumes and cover letters for every single company, concentrate on the ones that appeal to you the most.
Find five good companies. Target a resume for each one, and then customize a cover letter to accompany each. The next day, find five more. Stop sending broad resumes and template cover letters to random companies. Your goal is to get a job that you really like.