I'll offer my opinion. I have a 3' x 6' brooder in the coop. One end is heated with a heat lamp, two heat lamps in cold weather. My typical brood is around 20 chicks, I've had as many as 28 in there. In the warm weather it is pretty wide open, in winter I wrap it in clear plastic. It still has decent ventilation up high. In winter I keep one end toasty warm but on some mornings I find ice or frost in the far end. The temperature is certainly not the same all through it. I find chicks straight out of the incubator are very good about managing their heat as long as they have options. In ridiculous heat waves like many of us have just had or are having I've turned the heat off at a couple of days.
This is a link to the heating pad cave brooder that was mentioned above.
Mama Heating Pad in the Brooder (Picture Heavy) - UPDATE | BackYard Chickens - Learn How to Raise Chickens
I've never used the heating pad cave or a heat plate. In research on heat plates one manufacturer (I don't know if it as Producer's Pride, it was most likely another) said how they calculate the number of chicks it will hold is to use day old chicks. How many a heat plate or any other brooder can handle will depend on breed and age. A four week old Australorp wil be a lot bigger than a one week old Serama. I can see why a manufacturer would have trouble coming up with a number of chicks it will handle so I'd have trouble believing their numbers.
Did Cackle tell you why they recommend a heat lamp? Some details could be informative. You may find that they don't apply to your situation.
@Tonyroo do you know why that brooder or heat plate caused mismanagement or what the problem was?
My perfect brooder is one that gives the chicks a place to warm up when they need to but also gives them a placed to cool down if they get hot. My brooder with the heat lamp works great for that but inside a house and with a small brooder it is very easy to overheat the chicks using a heat lamp, plus heat up your house and make your AC work harder. If I were brooding in the house I'd really research a heating pad cave and heat lamp.
Several years ago a lady that sometimes used heat lamps and and sometimes a heating pad cave in her coop compared the two. The only thing I remember from that comparison was that the heat lamp could handle more chicks than the heating pad cave. She was raising CX meat birds which grew really fast.
@TaylorGlade to me the biggest difference in picking them up from TSC and the post office is that you know when you are getting them from TSC and they should have already at least somewhat recovered from shipping stress. It is always possible for them to be delayed a day in shipping.
I hatch most of mine that go in the brooder. It is extremely rare that I lose one. I've probably received about 80 shipped from a hatchery in three different shipments. I've never lost one of those. I do not get any from a feed store. As long as the mailed chicks arrive on time they do great.