Should a letter of recommendation be customised for each university when applying for a masters program? I've just graduated in Brazil and I plan to apply in some different Master courses in US. I plan to apply to different universities and I did not want to ask my professors to write many different letters. 

Should the letter of recommendation be specific to the university I am applying to or  can I use a generic letter for all applications?
Never hold your own recommendation letter (looks unethical), let your current lecturers send the recommendation letter to the universities directly (via email for example), that you applied for.

Most universities, ask for two recommendation letters, by asking the information of the lecturers (e.g., email, address, phone, etc.) who do that for you. So the recommendation letters will be sent by your lecturers (e.g., email, mail, online form, etc.) to the universities you applied for; without you seeing/holding the letter.