Importing png sequence After Effects

Based on your other comment, you'll probably want to import them as separate images, but you can then automate some of the tasks needed to arrange them on the timeline. 

Import all images into AE. If they're already named in such a way that AE wants to import them as a sequence, make sure to uncheck/disable that option when importing them. (It'll be in your file import pop-up).

Create a composition as the desired size, or use one of the images to create it. Make sure to be mindful of the duration when creating the comp. If you're creating the comp based on one of the images, make sure to change the duration to the desired length.

Add all images, in the order you want them to be in - i.e. #1 is the top layer and #20 is the 20th layer, top-to-bottom.

Select all images (CTRL/CMD+A or click the top layer and Shift-click the bottom layer), and trim them to the desired length (one frame, two frames, etc.) using the hotkey ALT/OPT + ]. 

Keeping the layers selected, right-click one of them, then choose Keyframe Assistant, Sequence layers. Make sure Overlap is disabled, as you likely won't want that here. 

You should now have all your layers easily spaced out over time, and can work your way through manually positioning/scaling them as necessary. 

If using one of the layers as a guide for the position/scale of the others, simply extend it out (either with your mouse, or by using the above-mentioned hotkey on a later frame, with that layer selected), and use an appropriate Blending Mode or reduced Opacity setting to allow you to see the other layers as needed.

there is no such thing. That's the whole point. A "sequence" in video processing has the same consistent specs for all its frames. You have to import the images separately and arrange them accordingly on the timeline e.g. using the "Sequence Layers" keyframe assistant.

Mylenium