Free mockup generators
Plenty of tools offer free mockups. What "free" means varies a lot. Here is what to look for, and what the limits usually are.
What "free" usually means
A free mockup tier almost always has limits. The common ones are a monthly cap on how many mockups you can make, a watermark on the output, and a lower download resolution. Some tools also lock the better scenes behind a paid plan.
None of that makes a free tier bad. It makes it a way to try the tool properly before you decide to pay.
The Mock House free plan
The Mock House free plan gives you three mockups a month, every template unlocked, and downloads at 2K with a small watermark. It is enough to see exactly how your work looks in a real scene before you commit.
When it is worth upgrading
Upgrade when the limits start to cost you time: when you need clean output with no watermark, 4K downloads for print or hero placements, or simply more than a few mockups a month. If you are doing paid client work, you also need a commercial license, which the paid plans include.