I've a table 37 rows x 10 columns (no header or footer rows). Even without any formatting or content there seems a random limit on how many cells I can select at any one time. I seem to be able to select the top third of the rows. Below this it's very random what it will allow me to select.
It won't allow me to merge rows together that straddle this two thirds point either.
Is the table simply too big for Indesign?
Additional information: If I attempt to add rows (in the dialoue in the toolbar) it adds tham at the same point where it limits me to either selecting the cells above only... or below only.
I don't do a lot of work with tables, but I've very successfully selected large tables over multiple pages in the past. One problem you might be encountering is that ID will not split a row across a frame or column break, so if there is a lot of data in a cell that might be a factor in your merge problem.
I think this happens when a large part of your rows are actually Header or Footer rows. You can see if that's the case by selecting them with the table pop-up menu. If so, convert them to regular body rows.