Basically I bought 4 tickets on SH a few weeks back, and sent 2 of them to my buddy's SH account. When I tried to list my remaining 2 tickets, it forces me to list all 4 of them. All done on the mobile app.
Very sorry for the trouble! You should see an option to edit the quantity when selecting Resell. The app is more limited than our full site so you may want to try on a desktop/mobile browser if you're having trouble making the change
Another option would be to create the listing from scratch by going to Sell Tickets at the top of the page and searching for your event. You'll just need to doublecheck the delivery to ensure you get the tickets to the buyer either via PDF upload, mobile transfer, barcode entry, etc!