How many eggs can a broody cover?

[SueT]

How many of her own eggs can a little Sebright hen cover? Is there a rule of thumb?
My Sebright Moonshine is my only reliable broody. She has started each April, so I am saving up eggs for her.
Last year she hatched 4 Spitzhauben eggs, covering them easily. I only have 1 pair of Spitzhaubens left, so would like her to hatch some more of their eggs. But her own eggs are also fertile, and I'd love to have Sebright/Spitz crosses. (He's my only rooster)
I was thinking of giving her some of each, but I don't even know the rule of thumb for bantams or LF hatching their own eggs....
Here's the comparison of Spitzhauben and Sebright eggs. The Spitz eggs are smallish.

Thanks in advance for sharing your advice/experience!
Sue

 

[chickens really]

I always try for 8. My biggest concern is her being able to keep the Chicks warm as they grow..