Regarding "Guardianship" I have filed papers with Michigan Probate Court. How many "original" letters declaring guardianship do I need?

Depends how much would be probated.

So far I've been able to manage with photocopies but I'm not done yet.

I’d get 6 as you’d be surprised who will want an original to do anything.
And they keep them...... Also if this is like other probate court actions doing it initially will be the cheapest & you wait right then & get them.

Take cash too for them. For some Courthouses anything check requires time to clear before you can get it and there a mail delivery fee and CC add a 2%-5% surcharge.

Good luck and good for you in getting this done on your own!

It's hard to know. Some places will accept a certified photocopy of legal documents, some want a Court original. Some places take their own photocopy and give you back your own document, some places want to keep it themselves. Sometimes it's much cheaper to get extra copies immediately, than to get the Court to search back through old files. Unless you have a good crystal ball, you don't know what might happen in future to need your to produce the document. Unless getting extra copies means you can't afford breakfast, I would probably get a half a dozen.