In my usage, splitting into smaller files hits diminishing returns quite quickly.
Splitting by "topic" can work, if the topics are mostly disjoint, with not a lot of transactions between them. Eg, separate files for transactions, account declarations, and prices. Or separate files for accounts which don't interact much with the others.
Splitting by time period makes sense if you accumulate enough data. And if you do it, I'd apply it consistently to all the files you already have, eg 2023transactions.journal, 2023accounts.journal, and 2023prices.journal. Otherwise if files have different lifetimes things tend to get muddled.
If splitting by time, I feel yearly is usually the sweet spot. Splitting monthly means the number of files you have to deal with will multiply 12 times as fast. It might be doable with discipline, but try it, I bet you'll find it's not worth it.
To answer your question, I currently keep 5 files per year - YYYY.journal, YYYYaccounts.journal, YYYYprices.journal, two others related to investments - and one more which doesn't need to be per year, future.journal.
I tend to use (Emacs) editor features to make these long files more navigable, like adding foldable section and month headings.