TSQL，如何只更新第一个可用桶，

I need to do this update to add another value to a series of buckets (B1-B5), but only 1 time into first available, like in sample below, can not come up with any solution to mark it once it filled, tried to use flag, but looks like it doesn't' work during the same tx. This is sample code which is not right yet. I'm on MSQL 2016. Thanks all for review;I need to do this update to add another value t