There's no set rule about how many characters there should be or what plots to use/avoid. Use as many (or as few) characters as necessary. Just make sure the characters serve some sort of purpose and aren't there for arbitrary reasons. Also, no plot is inherently good or bad. If you write it well, then any plot can be a success. So, that being said, the only plots you should avoid are those you feel you aren't capable of writing.
