If you're talking about the wall adapter it's printed on the back of the adapter. I think it's 400 or 500 mAh. And that adapter is universal for all of the Streamlight rechargeable cradles except the Stinger Fast Charger when it is used with the UltraStinger.
If your reference is to the current output at the charging sleeve, that will vary depending upon the SL20 model that you are charging. The SL20XP will draw 180 mAh, the SL20/SL20X will draw 220 mAh. That's documented in the operating instructions that were packaged with your SL20 product.