30k would be a good start, I think. MotoGP gets somewhere in the 50-70k range for the race, maybe? Saturday looks like it could be awfully rainy, but Sunday looks good. The weather at COTA in March can be hit-or-miss, as what's now known as Blancpain GT America has shown over the last few years.
The race should have more 'action' than F1 usually does, but it's still a very large circuit for IndyCar - one where the field could get strung out easily. It'll be interesting to see how that appears to the crowd.