After googling around as well as browsing this forum, I failed to find the solution to get gapless sequential ID:s from a TeraData system - is there one? Conflicting information on usefulness of e.g. Identity was offered. In case my post has already a detailed how-to answer somewhere on these forums, please point me to it. Thanking you all for your help already.
The only way to get a gapless sequence is to do it on your own with ROW_NUMBER like
row_number() over (order by whatever)
+ coalesce((select min(seq) from tab), 0) as seq
Instead of "select min(seq) from tab" you might have an extra table storing with the current max value for this table and then you increase it by the activity_count (probably using a SP).