Flat cookie sheets without a rim along the edge actually help cookies bake better as they allow air to freely circulate over the cookies. Cookies bake better by ensuring uniform heating and baking.
You should always try to bake one sheet of cookies at a time and centre the rack in the middle of the oven as this will help them bake better.
If you place two baking sheets of cookies in oven at one time, it’s best to switch their positions halfway through baking to allow for more uniform cooking.
Always try to allow cookie sheets to cool thoroughly between batches as putting the dough onto a hot cookie sheet may cause the cookies to spread and thin on the edges causing the edges of the cookies to bake too much.