I do not believe that there is any research that states how many pumps a unit should have. It is really a matter of looking into your unit to see what is needed. For instance where i work at we have:
2 pumps for the L&D unit - mothers tend to stay in this unit for about 2 hours before transfering to the MOther Baby unit. If the baby went to NICU the mother can start pumping within these 2 hours. Mothers who also are on Mag after birth stay in the L&D unit - we try to keep the baby with them as long as their is a second person there. If not the baby has to go to NICU but mother would be able to pump if needed. 
In the NICU there is 4 pumps. Mothers can pump anytime they visit their baby. The pumps are all mobile and will be brought into the infants curtain so mother can pump at bedside. 
In the mother baby unit there is about 10 pumps. We have never actually needed all 10 pumps at one time but there is plenty for any reason needed. This is also where the pumps that need to be borrowed comes from as well (ER, ICU, ect). We also provided 'boarder status' to mothers whose infants are in the NICU. As long as we have the room mothers are allowed to stay the entire time their child is in the NICU. 
In the pediatric unit there is 3 pumps. At least one of these pumps are placed in a non-occupied patient room for the use of nurses in the hospital that may need to pump. 

We have a level 2 NICU and delivery do between 3000-3500 deliveries a year. 

So you really have to look at your own hospital and determine all the times a pump may be needed. 

Jessica IBCLC, RN
NC, USA

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