Include a small icon button next the Upload and Create buttons. This button would default to 2x (which creates two clips). Allow users to click button for 4x, 6x and 8x so that multiple extensions can be made at once.
Personally, I always have 4-8 clips before I decide which one I like the best.
At moment there is a delay between ability to click Extend/Create/Remix so this feature would save time and waiting.