OK, Thank you.
What is the reason that the number isn't the same for all the album tracks ?
The tags make all the difference in how the albums are displayed in MB. If it is just one disc in the album, then the disc number should absolutely be the same for all tracks, and that could be your problem. In fact, for albums with only one disc, that disc number is most often left blank; that way you see the track number as "1" instead of "1-1". Why is it different? Who knows - some strange setting in how you ripped the disc? Or maybe you accidentally entered a disc number when editing the tags on just one track for an album that previously did not have any disc number?
Try this: Right click on the album, so that all the tracks are highlighted, then select Edit to see the tag editor. Any field that shows "mixed" has more than one value somewhere in all the selected tracks, and, depending on the display and sort options you used in the MB panel you are looking at, can certainly cause an album to be "split" or to see tracks "missing". For example, if the album name is changed by just one character, even a space, or the year is different/missing, it will cause the track do be displayed as a separate album.
If you find the problem is being caused by variable tag information that should really be the same for all tracks, it is a simple matter to fix by just highlighting all the tracks where a tag should be the same, then using the tag editor to change them all at one time by entering the correct tag info. This will bring them all back together in the album display.