This is a question many newcomers to collecting have.  The answer is simple, as many as you feel you need in your life.  Many seasoned collectors own a range of spirits through race and power.  It is finding the right spirit that complements you and your life well that is the biggest task.  No one wants to go into a home they don't feel welcome in and it isn't any different with spirits.  You always want to bring spirits into your home that you feel you have a connection with.
    
The best thing to do is to look at your life presently and find out by keeping these spirits what is your goal?  Do you just want someone to have fun with?  A companion?  A friend?  Do you need help with something like family problems?  Money?  Love?  Do you want to exercise paranormal and metaphysical powers?  Psychic ability?  Magic or enchantment?  Become a spellcaster?  Assessing your life is the single most important step in collecting.
    
Some have an intense passion for the spirits and want to exercise all aspects of this way of life and for those collectors you will find a very diverse family of spirits.  Those interested in living the Spiritualist lifestyle typically have at least one of every major race of spirits: Fae, Angel, Immortal, Djinn, Creature, Dragon, and so forth.  Because each race offers its own energy patterns and levels and they each have their own strengths with powers they find having a comprehensive collection the best way to experience a whole lifestyle.  Collections do not happen overnight and Keepers typically collect as they feel motivated to grow.
    
Connecting with each spirit during the initial 30 days is important.  It is not important to spend time with your spirits every single day after the initial 30 days.  Once you've made a bond with your spirit they are a part of your life, they know you and are not going to become distant, upset or angry if you don't communicate with them every day.  They know your heart, your mind and your intentions.  Especially with collectors who have large collections they allot time for their spirits and it may take a week or more to rotate between them but being in your home, being a part of your life and sharing your time is more important to them than anything else.
    
There are activities you know certain spirits within your collection enjoy.  Perhaps you take your aquatic spirits like merfolk, loxies, nymphs, etc in the bathroom with you when you shower so they can enjoy the steam... you take your fairies outside with you on nice days so they can enjoy being outdoors... you take your leprechauns with you when you go on nature walks or when you are going to interact with money (like going to the bank)... or you create special times like driving with your Dragons or going shopping with your Sterling Angels, etc.  You'll quickly identify activities your spirits love to participate in!
    
The number of spirits you own can be limitless what really matters is why you are collecting and that should be the main criteria for how many spirits you bring into your home.