I have a long growing season. And I often order sweet potato slips.
I plan so many per bed, then I let them just fill in the bed.
Once the bed is full, they have months of growing, and I have many more "hills" of sweets growing. And I don't particularly like the huge monsters, I prefer them a reasonable size. And with months and the beds dug down 2' well, I could get monsters. So consider your season, your bed and depths. I had sweet potatoes even get under the tiles of the bed... interesting to get those out.
Hey, I learned something in 2009. I got some sweet potatoes at the grocery store, they would not sprout at all. No matter what I did, they rotted. Hmm? Anyhow, towards the end of summer I bought a case from the local fruit stand, they sprouted like crazy! Guess what I found out... early picked sweet potatoes will sprout. But, after they have been stored awhile, or the ones the store wants to sell later... are treated to prevent sprouting!
I also, picked up some of my sweet potatoes and kept them with the roots on them, and brought them inside, no more hassels getting slips... They are growing as a houseplant, and I simply snip off a limb and root it. I have constant starts! If I don't kill them, like most houseplants! ha ha
If you must use sweet potatoes from the grocer, don't get them in the winter, get them in the fall, and save them yourself. Every vendor I could find had too wet of a year and offered no sweet potatoes in 2009, finally, a Texan friend, sent me some in the mail... they travelled great!