Is there a limit to collection size?
No, not to my knowledge. Collections have always been troublesome in Bridge and you must not rely on them solely If you have put a lot of work in a certain collection also use either labeling, renaming but most easiest is add a keyword to all the collected files so when in trouble you can always revert to it via the 'back door'
Meanwhile check if you have no strange characters in the filename like ' or / etc because that could be also causing troubles.
Also check if you have the correct read and write permission for the files, folders and disks you are using.
And maybe try a reset of the preferences which can cure the strangest of problems.
I've had over 9000 in a collection.