What’s the alternator inspection interval for 912iS?

That info would be in the 912is Line Maintenance Manual (LMM)

Do you mean the External Auxillary Alternator?

LMM Chapter 05-20-00, Page 13, Item 11. Auxillary Alternator  (100hr)
Which refers to
LMM Chapter 12-20-00, Page 17, Checking Belt Tension. 

OR

The Internal Generators?

LMM 05-20-00, Page 11, Item 7, Visual Inspection (200Hr)
Which refers to
LMM Chapter 12-20-00, Page 47, Item 3, Check of wiring.

Thanks, Bill! I don't think mine has the external auxiliary alternator. How could I double check that? Is there somewhere that would be on the engine that I can look at and if it's not there then I know it doesn't have it? What's the purpose of the external auxiliary alternator?

What are the internal generators and how many does the 912iS engine have? Sorry for the stupid questions. I come from the Continental engine world! So with Rotax engines, specifically the 912iS, there's no 500-hour alternator inspection like you have with a Continental engine? 

The external Alternator is driven by a belt from the back of the propeller.
You most likely do NOT have or need one.

One generator is associated with each Lane.
The A generator (~10 Amp) normally powers the engine.
The B generator (~30 Amp) normally powers all the panel loads and is the backup for the A generator.
Both Gens are located on the crankshaft, internally within the engine crankcase. Nothing to see.
There are NO Magnetos; if Both Gens fail, the engine will run only as long as the battery lasts. (~20-30 minutes).

This is a computer-controlled engine.
It has very little in common with the legacy Continental engines you may be familiar with.
You need to download all the engine manuals and read the Operators Manual First.