I'm making a sequence of 68 images in 12 seconds, but when I export it to "PNG Sequence" only 2 images are sent to output.
Why is this happening?
You have set an In/Out area (the blue, vertical highlight in your Timeline) and you have told Hitfilm to only Export the In/Out area, which is two frames duration. Hitfilm has done exactly what you accidentally told it to do.
Delete the exported frames from your output folder.
Put the Playhead at the first frame and press "I" on your keyboard to set a new In Point. Move to the last frame and press "O" to set an out point. Then re-export.
In general, look at the Export button settings. There are two options. "Content," which Exports everything on the Timeline, and "In/Out" which only exports the In/Out region you have set on the Timeline. Get familiar with these options. Be aware you might have set In/Out as the default behavior of the "Export Now" command. These are controls you can familiarize yourself with later. By setting In/Out points to your entire Timeline it won't matter which Export you use, so you can render now and worry about these controls on your next project. But, in the long run you want to be familiar with the options. We had another user this week wondering why his Export was longer than his Timeline. He'd accidentally set his Out point three seconds after his last clip and had Exported In/Out instead of content, so, in his case, he had blank black at the end of his render.