How much info do I need to give management co.?

It's safe to assume that whatever info you provide will not be well protected. Real estate agents, boards, and the like are horrible at protecting PII.

As for what's "acceptable", this varies by person and agency. You can always try for the minimum and see if that works. For example, for income, you could ask your employer for a letter stating your position, compensation, and tenure at the company. This will reveal far less than a tax return.

I believe SSN is needed to run a credit check. You may not be able to avoid giving that to the management company.

Thanks. I'm semi-retired and freelancing so can't get a letter. I think I will try redacting, and if they want to see more, I could show it to them some other way I guess, or look for some other rental that doesn't do things that way. The site doesn't even say it's secure.
They seem eager to rent out apts. and are offering two months free rent for the first year, so maybe they'll be lenient about whatever I show.

I didn't know they were notorious about that! Good to know!

I had a giant fight with a celebrity client once when I refused to use an online portal for her application, because I thought that there was a high chance that the info would be leaked. FWIW, many managing agents will take a paper application, but it will take longer, possibly a couple weeks longer, for them to process.

Interesting, thanks! Funny that she wanted to use it--but if it had leaked I'm sure she would NOT have been happy. :)

They want to make sure that you can afford the apartment
The less you info you provide, the less likely you are to get accepted

Well if I am showing the number that is my income for the year, do they really need to know each individual source of it? Not sure any of the sources really prove regularity of it or not, and I can show roughly same amounts for consecutive years, though they don't at all specify what they want.