How many stepper motors can a motor shield control?
There are motor shields that allow for running two stepper motors, like this one http://www.aliexpress.com/item/L293D-Motor-Drive-Shield-dual-for-arduino-Duemilanove-Motor-drive-expansion-board/32341780904.html?spm=2114.01010208.3.1.PA9IVu&amp;ws_ab_test=searchweb201556_9,searchweb201602_2_10017_10005_10006_10034_10021_507_10022_508_10020_10018_10019,searchweb201603_9&amp;btsid=9d220772-7d0a-437d-8534-651a85644de4

Or there are boards that are smaller than a shield that you could use, like these:  http://www.aliexpress.com/item/28YBJ-48-DC-5V-Five-line-4-phases-Stepping-motor-Deceleration-stepper-motor-Fuses-length-25CM/818857514.html?spm=2114.01010208.3.1.LZ80QW&amp;ws_ab_test=searchweb201556_9,searchweb201602_2_10017_10005_10006_10034_10021_507_10022_508_10020_10018_10019,searchweb201603_9&amp;btsid=201933bf-6385-4a45-a3c2-01236c01cd7a

But you will need to look at the specs to make sure they fit your needs, the best way to do that it to look at the main chip that is driving the motor and look at what that can do.

Hope that helps.