I'm wondering if there is a certain amount of bytes/second of bandwidth I should use as a guideline for "the number I don't want to exceed".
Currently when I play with 2 players being synced with NetworkInterpolateTransform, its about 2200 bytes/second. I'm wondering how bad its going to get once I introduce enemy AI units; if I have 10 units its going to be something like 12,000 bytes per second I imagine. How much is too much?
I guess it depends on what connection you want to support. 12,000 bytes per second is about 11.7 kB/s. For example 0.5 Mbit ADSL support theoretically 64 kB/s. It means it would take about 183 ms to send that data to a 0.5 Mbit connection. It might be too costly, or it might be fine if you're using good dead reckoning schemes. I haven't done multiplayer in Unity3D so I can only draw upon my knowledge since university and apply some deduction. If your network code would be running in 30 fps that would mean that each update should be at most 2.1 kB to avoid choke.
Or another way of putting it; you're consuming 11.7 kB/s of their 64 kB/s connection so you should be able to sync every frame under perfect conditions.
What is your target?
most games target 128kbps connections as their lowest connection speed and some of them as you said take 512 as their connection speed limit. you should take another factor into account. you have RPCs too for sending important events like shots or ... they take more bandwidth and if they are common like shooting in an FPS game you should take them into account too. also keep in mind that packets sometimes drop so other than using prediction and dead reckoning you should try to make your game's connection requirement as low as possible. it should be high enough to allow smooth gameplay and as low as possible.