Hi, if this is for your baby sweater you probably have a pretty small neck opening (although remember babies’ heads are big and often they even use a shoulder placket or some kind of opening even on a pullover for babies). In order to use one circular needle you have to have a short enough needle so that the number of stitches you have will reach all the way around the needle enabling the ends to come together. The most common small circular is a 16" one, but they have smaller ones 11" and I have even heard of 9" although have never seen one and can’t imagine using one. If you don’t have a small enough circular you can use 2 circulars of any length. Pick up half of the stitches on one of the circulars and the other half with the other one. The only trick is to knit the stitches off one circular with the same circular, don’t work from one needle to the other.
You don’t have to sew a seam if you work in the round. There are free videos of how to use circular needles to knit in the round on this site under “Advanced Techniques”. You basically pick up all the stitches and then instead of turning around to work the other direction after you have picked up the stitches, just keep working around the circle so that the last stitch you picked up and the first stitch you picked up are right next to each other.
You can also do a neck on double pointed needles, and even with one longer circular doing something called a Magic Loop. I am new to Magic Loop and have never used it this way although I’m sure it would work. There is information about all these techniques in that same video section.
You could also pick up the stitches on a circular and work back and forth and sew a seam in the neckband. I like to avoid the seam, but if you had to you could do it that way.