From this site and probably fairly close:

155 WB
784 NB

939 Total Aircraft in fleet.

I am not sure how this compares to UA and DA but I think the percentages will pretty much stay the same as they have in the past decade or so. Our 787 and 330 CA spots have been slightly more junior than the 777. While there are some out of seniority spots because of the merger seniority integration the WB captain slots stay at the top seniority wise. As of this summer there are 1147 Group 4
WB captains. Total Jobs are at 14917. So basically you would need
some 12770 people above you to retire. With predicted retirements at age 65 that would be end of 2039 or so, 19-20 years away for someone hired today. For reference that 1147 seniority now is a 1986 hire. Over thirty years to get Group 4 captain and that generally fits with what I see on the line. Maybe like today most people are the same age plus or minus 5 years so when you get hired in the cycle is a really big factor. Hope this helps. (BTW info used from Sen list 2016)