So she will have:

2 less than a year
2 between 1 year and 16 months
1 preschooler

PLUS a 16 month old in the fall? Are any her own children?

I think that everyone has their own comfort zone. For myself, I restrict myself to 3 under 3 because I know that's where my comfort zone is at the moment. However, that is only right now, while I've got two infants. When those infants are toddlers, I could easily handle 5 toddlers/preschoolers.

Is it possible to sit in on her care for an hour? The children won't be as behaved as they are when there aren't strangers present, but it could give you an idea of how she handles her time with all those young children.

All the best,
Emily