You can find options for the project-wide appearance, enclosures, and contents of bar numbers on the Bar Numbers page in Engraving Options.
The options on the Bar Numbers page allow you to choose whether repeated sections are included in bar numbers or not, change the case of letters in subordinate bar numbers, and change the minimum gaps between bar numbers and clefs shown at the start of systems. You can also change the line thickness, size, and padding values for bar number enclosures.