To assist with answers-
Are you using Lightroom-CC (Cloud version) or Lightroom-Classic-CC (Desktop version), and the Version Number?
Do you shoot raw or JPG with the G15?
What are your options in the Export dialog? (File-type, etc)
What do you mean by "exported files start with image # 16" ? Is that the 16th image in your Collection or something else?
If the export dialog shows you are exporting 60 images then you have selected 60 images.
Re-Select the images you want to export.
I am using LR Classic CC desktop version, 7.3. Most of the files are RAW and some are JPG. The 16th image in the collection is the first that is actually exported. I have repeated the selection process several times - that does not help. I have been using LR for years and have exported files from the G15 camera before without an issue. Export settings are jpg, sRGB, limit size to 800K (I have modified this limit with no effect); no image sizing or output sharpening; the only metadata.is copyright info. Again, all the images taken with the 6D camera export flawlessly to the designated folder on my hard drive. It's just the G-15 files that act as if they are being exported but never make it to the destination folder..
First I might suggest-
Update to Lr-Classic 8.0
Reset your Preferences file. How to set Lightroom Classic CC preferences
In the export dialog- remove the Size limit option altogether- remove the check mark- as a test (if not already tried).
If the G15 files were marked as 'missing' they would also appear as if exporting, but you should get the error message- "Some export operations were not performed"- Have you seen this error message?
Reset warning dialogs in Preferences / General - Prompts - [Reset all warning dialogs]
Perhaps some screen-clips of the G15 photos showing Info in Grid view, and your export settings.
Otherwise I am as baffled as you!
Updating to LR Classic version 8.0 solved the problem of exporting Canon G15 files.