There are a number of published photos of Dylan apparently listening to acetates of Blonde On Blonde in a hotel room on the European leg of the tour. Some of these show discs with only two or three tracks on a side, clearly not corresponding to any of the sides of the released album.
This is probably because many acetates were cut at 45 rpm, so would have a shorter recording time per side than a 33.3 rpm disc. Furthermore the acetates were generally single-sided, so the whole album could have been spread across six or seven discs.
Other Blonde On Blonde acetates are known to exist which do play at 33.3 rpm and contain a full side of the album.