In writing business letters, when we don't know the name of whom we are writing to which words are better to use?
Can we use "To whom it may concern"?
Yes, that is fine. However, it is very formal, for example with letters having to do with legal matters. Less formal and more typical is "Dear Sir or Madam:" (note the use of the colon; "To whom it may concern:" also should use one). If you know the title or job position of the individual to whom you are writing, you should use that: "Dear Judge:", "Dear Claims Adjustor:" and so on. Also, if the letter isn't about business, for example you are inviting the Claims Adjustor to a party, you would use a comma: "Dear Claims Adjustor," would be the style in this situation.
There is nothing wrong with writing, "To who it may concern", but personally it sounds very unprofessional and incorrect because it concerns a whole lot of people, including you who is writing this letter. SO I recommend that you use "Dear Sir/Madame" and of course remember to use your colon ':'at the end of the salutation
When you don't know the name of the person in charge of sponsorship, "Dear Friend," is both professional and friendly without being overly familiar. Also "Dear Potential Sponsor," which is a little more direct but sponsors already know you want money so they prefer you to be upfront.