Numbers on a Minecraft circle are a way to talk about the picture. This page sorts the blocks by their angle around the centre, the standard atan2 order, and writes 1 on the block with the smallest angle. That block sits toward the left side of the grid. The numbers then walk around the outline. They are not item IDs, they are not the order a well-known circle chart uses, and they are not a claim that block 1 must be placed before block 2. Untick the box and the silhouette is the same circle without the labels.
Radius 8 and thickness 1, which this page opens on, is 44 blocks. With numbers on, those are labels 1 through 44. The row table underneath is a different count: how many blocks sit on each east-west line. Use the row count to place a line, and use a number when you are telling someone where you stopped. "Through 12, then the axis" is the sort of sentence the labels are for. A chart that numbers a filled disc, or that starts at the top and proceeds clockwise, will not match these figures. If the chart is the one you are building from, follow the chart. This page will not pretend to be it.
Thickness changes which blocks exist, and the numbers follow the blocks that exist. A thick circle numbers every cell in the wall, still by angle, so two blocks on the same ray can get neighbouring numbers or not, depending on the angle sort. If you wanted a number only on the outer edge, set thickness back to 1. The thickness page is the one that starts at 3. The stair page replaces numbers with S and a facing letter, and it only does that on a one-block outline.
The PNG paints the numbers that are on screen. Download it before you change the radius, because the file is the current grid, not a stored plan. The centre cell is marked as the origin even when the thin outline does not fill it. Count the radius from that centre: eight blocks out along a straight row, and the eighth block is part of the ring. The whole width of that row, including both sides and the centre, is seventeen blocks. Numbering does not change that width.
Very small radii still look square. A radius of 2 does not have enough cells to suggest a curve, and the numbers will sit on a diamond or a plus. That is the grid. Start nearer radius 6 or 8 if the ring needs to read as round in a screenshot. Ovals are numbered the same way, by angle, once you switch the shape. A sphere numbers the layer you are looking at, not a 3D index. Change the shape only when you mean to leave the flat circle.