Numbering is increased every time a record is opened.

Every time a Contract record is opened, the Counter of the number will be increased. This leads to gaps in numbers and increased numbers very quickly.

Upon investigation, I found that this is happening due to the Embedded List for Contract records under the "Contracts" tab of your form.
Out of the Box, when new records are created the numbering is increased by 1 even when not fully saved. The same case applies to Embedded Lists.

To fix this, we have two options. We can remove the Embedded List from the form or if you want to keep it there, we have a System Property to enforce the next number to be used without any gaps.

The steps for both options are below.

NEXT STEPS (Remove Embedded List):

1. Navigate to Contract > Contracts > All
2. Open any record
3. Right-click the form header and go to Configure > Form Layout
4. Look for the "Form view and section" area and go to the "Contracts" section
5. Remove "Contract->Parent contract" from the selected list
6. Click Save

NEXT STEPS (System Property):

1. Navigate to System Properties > All Properties
2. In the Name column, search for "glide.itil.assign.number.on.insert" and go to the record
3. Set the Value field to "true"
4. Save the record