I will follow algorithm described in the following blog, but will have to modify some parameters for your case. You can also drop FillingTransform
from the blog code because you don't have holes in your regions.
rbc = ImageCrop[ Import["https://i.stack.imgur.com/wOJps.png"], {300, 340}];
b = DeleteSmallComponents[ColorNegate[Binarize[rbc, .81]], 50];
distT = DistanceTransform[b, Padding -> 0];
marker = MaxDetect[ImageAdjust[distT], 0.02];
w = WatershedComponents[GradientFilter[b, 3], marker,
Method -> "Rainfall"];
cells = SelectComponents[w, "Count", 500 < # < 3000 &];
measures =
ComponentMeasurements[
cells, {"Centroid", "EquivalentDiskRadius", "Label"}];
Show[rbc,
Graphics[{Blue, Circle @@ # & /@ (measures[[All, 2, 1 ;; 2]]),
MapThread[Text, {measures[[All, 2, 3]], measures[[All, 2, 1]]}]}]]

Then bare number of circled cells can be computed with
measures // Length
16
ComponentMeasurements
should be more useful thanSelectComponents
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