Is there a limit to how may pages you can write?!

You can't add more page directly but you can keep on adding measures and MuseScore will create pages. I saw scores with 100 pages.

A an experiment I created a new score with 3200 bars and used the "Break every X measures" Plugin to break every two measures thus creating, as it turns out, 160 pages. I then altered the spacing to four staves per page and this increased to 400 pages. Adding in most of the brass section (and shrinking the page to accommodate them) took it up to 1600 pages. Appending 999 measures really slowed things down and took it up to page 1646 by which time I realised that things were getting silly, particularly as I couldn't find a GoTo to get to a certain page.

The longest real-world score in my collection is 440 pages comprising the Hummel Trumpet Concerto obtained from a midi file. This results in some long measures of very small notes (trills get transcribed from the midi as 64th notes) with some pages only having one measure but even cleaning it up it will still be quite long and the important thing is that it seems to play normally and is completely editable.

I suspect that there will be a theoretical max. on certain objects but further suspect that that will be a very large number. No doubt in the source code one could look for the smallest potential variable class and its size.