Under the assumption the OP means sequences of standard good play for both sides* John's answer in the tens of thousands seems sensible. But how about standard "after joseki" sequences? How many of these are included in that 20,000 figure for
The Great Joseki Dictionary. For example after this common joseki:
- Click Here To Show Diagram Code
[go]$$B
$$ . . . . . . . . . . |
$$ . . . . . . . . . . |
$$ . . . . . . . O . . |
$$ . . . . . . X . . . |
$$ , . . . . . X O . . |
$$ . . X . O X X O . . |
$$ . . . . . X O O . . |
$$ . . . . . . . . . . |
$$ --------------------+[/go]
Here is a continuation for white:
- Click Here To Show Diagram Code
[go]$$W
$$ . . . . . . . . . . |
$$ . . . . . . 5 . . . |
$$ . . . . . 3 . . . . |
$$ . . . . 4 2 1 O . . |
$$ . . . . . . X . . . |
$$ , . . . . . X O . . |
$$ . . X . O X X O . . |
$$ . . . . . X O O . . |
$$ . . . . . . . . . . |
$$ --------------------+[/go]
Black could also tenuki instead of
. Does that count as a difference sequence? And another choice for white:
- Click Here To Show Diagram Code
[go]$$W
$$ . . . . . . . . . . |
$$ . . . . . . . . . . |
$$ . . . . 5 3 . . . . |
$$ . . . . 4 2 1 O . . |
$$ . . . . . . X . . . |
$$ , . . . . . X O . . |
$$ . . X . O X X O . . |
$$ . . . . . X O O . . |
$$ . . . . . . . . . . |
$$ --------------------+[/go]
And black could now hane or extend (or even cut perhaps). Another idea for white:
- Click Here To Show Diagram Code
[go]$$W
$$ . . . . . . . . . . |
$$ . . . . . . . O . . |
$$ . . . . 1 . X . . . |
$$ , . . . . . X O . . |
$$ . . X . O X X O . . |
$$ . . . . . X O O . . |
$$ . . . . . . . . . . |
$$ --------------------+[/go]
And for black:
- Click Here To Show Diagram Code
[go]$$B
$$ . . . . . 1 . . . . |
$$ . . . . . . . O . . |
$$ . . . . . . X . . . |
$$ , . . . . . X O . . |
$$ . . X . O X X O . . |
$$ . . . . . X O O . . |
$$ . . . . . . . . . . |
$$ --------------------+[/go]
or
- Click Here To Show Diagram Code
[go]$$B
$$ . . . . . . . . . . |
$$ . . . . . . 1 O . . |
$$ . . . . . . X . . . |
$$ , . . . . . X O . . |
$$ . . X . O X X O . . |
$$ . . . . . X O O . . |
$$ . . . . . . . . . . |
$$ --------------------+[/go]
or
- Click Here To Show Diagram Code
[go]$$B
$$ . . . . . . . . . . |
$$ . . . . . . 3 1 . . |
$$ . . . . . . . 2 . . |
$$ . . . . . . . O . . |
$$ . . . . . . X . . . |
$$ , . . . . . X O . . |
$$ . . X . O X X O . . |
$$ . . . . . X O O . . |
$$ . . . . . . . . . . |
$$ --------------------+[/go]
or
- Click Here To Show Diagram Code
[go]$$B
$$ . . . . . . . . . . |
$$ . . . . . . 6 1 . . |
$$ . . . . . 4 . . . . |
$$ . . . . 5 3 2 O . . |
$$ . . . . . . X . . . |
$$ , . . . . . X O . . |
$$ . . X . O X X O . . |
$$ . . . . . X O O . . |
$$ . . . . . . . . . . |
$$ --------------------+[/go]
And so on and so on. I could easily make a tree of common continuations after this joseki with several dozen leaf nodes. Now this joseki is perhaps particularly rich in the breadth and depth of standard continuations (though of course me being ignorant of standard continuations in other josekis, or other continuations in this one, doesn't mean pros/other players don't know them!) but it wouldn't surprise me if including these 'after joseki' standard sequences could bring the total of "corner sequences" up to 50,000.
*rather than just some combinatorics of possible number of legal moves which would be utterly huge: as a first approximation we could say a corner is 9x9 and 81! is 5797126020747367985879734231578109105412357244731625958745865049716390179693892056256184534249745940480000000000000000000