First, are your hens broody? If they are not, then they will not sit on the eggs, even if you want them to.
Also, depending on where you live, it may not be a good time for chicks. Right now we have five inches of snow on the ground, and temperatures occasionally going into the negatives. Not a good time for chicks.
As for size. Most hens can sit on around twelve eggs. Now, do consider that they need a well made nest, and that the twelve eggs are the size that they themselves would lay.
To have a well made nest I suggest a box around one foot by one foot. Make a bowl shaped area out of staw. This will help all the eggs stay exactly where they belong, under the hen.
Egg size plays a huge factor in how many eggs a hen can sit on. If you are shoving Rhode Island Red eggs under an little Old English Game Bantam you may only be able to fit four eggs under her. If you are putting Old English Game eggs under her then you could fit many more. Since you are guessing how many eggs will fit, you can get the hens a sane amount like 5-6 eggs each.
Also, from past experience you may not want largefowl hens to sit on bantams. I tried hatching some Silkies under my largefowl Brahmas this year, and they crushed all the eggs, and chicks.
Happy hatching,
Jamie/Rhode Runner