Are you talking about the display of a text table? If not, Tableau will support as many columns as you want with the caveat that columns coming from the database will be limited by the relevant driver - for example 256 columns from Excel...
As for rows, there are people with workbooks that have 100s of millions.
It seems he is asking about the display of a text table. 16 is the max. You can find ways to work around this by putting two sheets side by side on a dashboard without headers showing on the second sheet, etc. But I think the reasoning is that with any more than 16 columns in a text table, we may want to ask ourselves: is there a better way to visualize this?
Agreed, though sometimes you need more than 16, especially if you are going to use the table as a source elsewhere (by exporting it). Scroll bars would help..