How do I know male or female cucumbers

I don't think you'll be able to tell at the seedling stage.

When they flower the male has no lump at the base of the flower. The female has a large round lumpy bit which will develop into the fruit. A pregnant lump, lol.

I think you need the males for a while to pollinate the females and then you get rid of them so all the energy can go into the females to produce the fruit.

If I'm wrong, then I'm sure an expert will correct me. :-)

im not an expert but in some varieties the male flower polinates the female flower and the fruits are bitter. thats why we are advised to remove the male flower. the female flower has, as you say the swelling behind the petals. it looks like a very small cucumber.

i wouldnt put more than 2 in a grow bag as they are hungry/thirsty plants. [and how many cucumbers can you eat?] :o)

Have a look at the seed packet - it should say whether it's a self-fertile one or whatever. I bought an F1 plant that the lable says I don't need to remove the male flowers from. With 10 plants, you'll be able to go into business lol