What is the minimum resolution to scan documents?

I used to use the scanners at the local U, but they're off-limits to civilians due to the coronavirus. Some have suggested that I use my 'smart'phone. I have an un-smartphone. Its 640x480 camera makes unreadable images. I also will soon need a bluetooth device to control an automated gate. Thus I want to buy a cheap 'smart'phone but not for making phone calls (remember those?) or browsing the Internet, just pictures and bluetooth. How low a resolution will allow me to scan my tax returns (for example) legibly? I won't get a SIM for it.