If your are applying for MS in US universities then application fee for an university ranges from 50 to 150$ and there are mailing expenses and lot of time associated for applying to any university. So you will need optimum strategy to get best possible school with minimum possible applications.
Usual strategy is apply for 3 ambitious + 3 middle level + 2 safe universities. You can change order depending on your profile. For example, if your academic score is low you might want to add 1 more university in safe category and remove from middle level.
Safe universities are those where you are sure to get admission based on your profile. You will put lot of efforts for study, preparing documents, applying to universities so you will always want at least 1 admit in your hand at the end of all this exercise. This will save you 1 season (fall/ summer) or possibly a year. Safe university will be usually low rank or costly, where few people with your profile will apply. So chances of admission will be high. Just make sure 1 thing before applying, if at the end this is the only admit you have will you go for it, if yes go ahead and apply.
Middle level universities are ones which have descent rank, descent chances for funding , good location. Chances for admission in these universities vary depending on various factors such as allocated seats for foreign students, aid from different agencies, focus area of university. For example, in 2007 many students who applied to University of Florida got admits with not so great profile because they had more seats for foreign students that year. As there are many factors which decide admission it is better to go through counselor or apply more of such universities.
Ambitious universities are your dream universities. Definitely apply few of these as you will do masters once in your lifetime and you don’t want to miss a chance just because you did not apply. Factors to consider are 1) Research that is done in that university and whether you can prove to them that you can be useful to them in some particular area. 2) Whether you have good recommendation from somebody associated with that university. 3)One more factor to be considered is course that you want to do your masters in. Many very good schools might have course that is newly added and so few people know about it. You can try your luck with such institutes.
Finally, as there will be lot of things to take care when you are applying don’t worry about which school you should go in case you get multiple admits. You can always do that once you have actual admits in your hand.It is always good to have multiple admits for the MS so that you can have multiple options for specialization, institute open and it will help you to take final decision.