The force bubbles end up being roughly similar to a regular steel wall in terms of strength, although smaller orbs are tougher to break simply due to the natural physics of cross-tensile strength. Things impacting them would bounce off them in a similar way to bouncing off a steel dome.
The orb don't burst outwards once breached, they simply pop like regular bubbles, causing no real disturbance except to things inside the orb. When an orb is formed, any part of it in contact with a solid will not form, allowing you to create domes over things with the ground as the contact. Once any solid object interfering with the orb's full completion is moved out of contact the orb will complete. Liquids and gases will be displaced by the orb's skin.
There's no real limit to how many orbs can be created, it's more of a "user can dedicate this much power to orbs at any one time" thing, which obviously improves as the user gains more stamina and energy and practice. You could generate 1 large orb to envelop a significant area, or many smaller ones.
Additionally, the orbs themselves are very light so if the user relinquishes control of them then anyone could push them around, as long as they're not anchored to a solid surface. However, when the user is actively controlling them he's able to move them as he wishes, or make them damn near immovable.