Summary
Number Response is similar to Text Response in that it allows participants to type a custom answer to your question, but it only allows numerical answers (numbers and mathematical symbols like .,+-).
Available to all users, with premium customizations coming soon for functionality like setting a min/max allowed value and analysis features like filtering and segmentation available on the Analyze tier or higher.
Here's how the Number Response block looks to survey participants (example shown is a Van Westendorp Pricing Study).