Information online states that a 18/36-bit DECtape holds 576 blocks. People claim it may be possible to add a few more blocks, but no more. Where does this number come from? Is there a hard limit on the length of the tape?
The standard tape reels had a capacity of 260 feet of tape. Standard tape thickness was 1.25 mil, so it would have theoretically been possible to put more tape on a reel if you made it thinner (which was hard to do because thinner tape was typically also stretchier). Making the reel larger wasn't a good option because the hubs on the drives were too close together to allow them to be much larger than they were already, and you would have needed two larger reels, one for the tape and a second one to use as the take-up reel.
The reason you could fit a few extra blocks on the tape was because both ends had a lot of leader tape so that you could securely wind it onto the take-up reel. You could therefore squeeze a few more blocks on the tape if you were willing to (a) reformat the tape, and (b) have a slightly increased risk of tape not winding properly because there wasn't enough on the take-up reel to hold securely.
I believe the 'hard' limit on the number of blocks would have been 4096 because block numbers were recorded as 12-bit values. This is assuming there's nothing in the electronics that would have limited it to something smaller, or which treated block numbers as signed values, either of which would have reduced the maximum number of blocks.
The operating system or software in use might also have imposed a limit, for example any system that treated the tape as a slow disk might assume the default number of blocks, and therefore be unable to use the extra blocks without modifications.