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Pressing the Enter key submits the formAsked by DestinyJoe on December 06, 2016 at 09:12 PM
Forms were designed that way (regardless if it's built in Jotform or not). This is something referred to as Implicit Submission as described in HTML5 specifications. This behavior doesn't just apply to your How many booklets field, it applies to the entire form regardless of what field is used (e.g. regular textboxes, one of our Quick Fields, etc.)
This is important to accessibility, assistive technologies and impaired users that cannot use a mouse at all. There are pages on the Web that are only usable if there is a way to implicitly submit forms, so user agents (browsers) are strongly encouraged to support this.
I found a really good read about this topic on this post in case you're interested to know why it was designed that way. At the latter part of that post will be alternatives on suppressing it (especially the Explicit Prevention part where they gave an example JS code snippet to prevent it).