In the olden days Filemaker only indexed the first 15-odd characters so that was all that would be used to match in relationships. But when you entered more charcaters it didn't break the relationship - it just didn't affect it after the character limit. Anyway, that's all old hat as the limit was raised way above that a long time ago.
But... are you really saying that you have fields in two tables which you want to use as Key Fields, and each could contain 4700 characters? Some sort of record, surely.
While Multi-value keys have their uses. It's far more flexible to store these values in a join table instead of in a single text field. In addition to not being limited to 150 values (I'm taking your word for this as I've never tried to use a multi-key with that many values), it's a design that is almost always much easier to work with than to try to stuff all those keys into a single field.
Thanks for the tip. I guess I'll create a join table for this instead of using a multi-value key. My solution was working fine until it went over 125 lines. I didn't know there was any limit until that happened. I think it would be helpful if this behavior were documented somewhere.
I updated my original post to include a screenshot showing the behavior. I hope this helps someone else in the future. I didn't test this with shorter id values. I wonder if shorter id values would somehow increase the line limit... that would be an interesting experiment.
In addition to the length, any such limit could be a factor of the RAM, the version of FileMaker and the Specific OS you are using.
I just ran a quick test using the numbers 1 - 301 for my keys and can see all 301 records in my multi-key based portal.
I wonder if another contributing factor in my case could be that my related records are coming from an external ODBC data source (SQL Server).
I know that this is not the answer to the question, but if there does turns out to be a limiting factor this may be another way around a similar requirement:
I created a Primary Table linked to a Secondary Table by Primary Key -> Secondary Key
The Primary Key in the Primary Table had over 60 characters. There were 5000 distinct records.
The Secondary Key in the Secondary Table had 10000 records, with 5000 of them matching the Primary Key.
Running the Go To Related Record script (match all in the current set) displays all matching records in the Secondary Table.
In weasel-words, "I had a list of (60 x 5000) characters and I wanted to show all the records that matched them" Sort of.
I did some testing in FileMaker up to 12500 lines with 60 character ids and could not find a limit. I think this must be a limitation only when using a multi-key field to relate to an external SQL table.