You'll need to specify the boundary of your numeric string. Assuming the strategy is right aggressive, you answer should look like this:
| rex mode=sed field=x "s/(26380[6-9]|26381[0-1])$/group1/g"
| rex mode=sed field=x "s/\d+/group2/g"
| stats count by x
The errors in your second expression take some nuance to explain, but you do not need any complexity. If your expression for the first group is correct, just use the side effect of mode sed: the second expression will never see numbers in the first group.