I'm decorating 4 cakes for some upcoming celebrations and I'm doing buttercream roae decorations. I haven't done them in a while but I need to make about 25 roses. Does anybody have any idea about how much buttercream I would need to make to get that many roses?  I'm making them a few days in advance of decorating the cakes and I don't want to make too much.  I'm not going to have time to make smaller batches as I go. 

Use a recipe of b'cream for air dried flowers.  It basically is the same with less fat in it.  Since I don't know what b'cream recipe you are using it's hard to advise you how to change what you are already using.  As to how much you need, it will depend on how big the roses are going to be but certainly you should have more than enough icing using 1 pound of powdered sugar to 1 cup of shortening.  Do not add any liquid - just slowly beat it for up to 5 minutes to make it nice and creamy,  You can add 1 Tablespoon meringue powder if you wish - it will help just slightly to make the flowers dry faster.   Try searching for it.

You also can use your regular recipe to make the flowers and freeze them.  When you are ready to decorate, take them out one at a time as they start to defrost instantly just from the warmth of your hands and the room temp.