Missing Numbers On The Number Line is a good introduction to missing numbers. You can project this on your whiteboard and practice and then have students use the worksheets later independently. This product contains 5 worksheets with number lines 1-10 and 5 worksheets with number lines 11-20. Students have to figure out what number the fall object is covering on the number line and write it below. See thumbnails for what the worksheets look like.

Fall Count The Room 1-10 & 11-20 Ten Frames
Fairy Tale Roll And Cover
Missing Number Worksheets -Winter
Missing Numbers 1-20 Circus