Hi dear Shu Ting Han

First of all, your image quality is not good enough for analysis. Share better ones

First method --> Zoom & count

In this method, you can zoom to your picture and count them with a multi-point tool. (You can activate multi-point by right-click).

It is easy and fast. Also, you can see your result from :

Analayze --> Measure

Second method

Separate color

You can increase separation quality with the Channels tool.

Go to :

Image --> color --> Channel Tool

Then you can activate or deactivate Red- Green- Blue Channels from that window.

With this technique, you can increase the contrast of your particle.

Then, You can count them Like the first method

In the end

After separation your image into colours, you can enhance brightness & resolution with two important tools.

First : Image --> Adjust --> Brightness/ Contrast

Second : Image --> Adjust --> Window level

Notice: You have to create a duplicate from the blue channel (Channel 3) before editing.

The final results have attached

With best Regards