Garmin 350 - How many lines?

I haven't tested the max, but I know for sure that you have...

Name of POI
Addr1
City, State Zip
Phone Number

So that is 4.

If you're talking from .csv input and POI Loader, then it's about 150 characters or thereabouts. The Nüvi 3xx truncates the description, for reasons best known to itself - and you can't access any other part of the display with csv input.

However - switching to .gpx format (but sticking with POI Loader), we can achieve multiple screens, each of about 32 chars by 12 lines. A proportional font is used, so it depends on the actual characters displayed.

The display format is

POI NAME
DESCRIPTION (truncated at 150 chars)
Contact (o/p if address present)
VARIOUS ADDRESS FIELDS
VARIOUS PHONE NO. FIELDS (sometimes icon instead)
EMAIL - mysteriously not truncated

See the following screenshots:
(These were created with GeePeeEx Editor, which swaps the email and description fields for the Nüvi 3xx series)
First screen
Scrolling down
Different font sizes and gml markup change thing a bit.

I've not measured the actual limit for the Nüvi 3xx series, but the StreetPilot C5xx finally gives up the ghost when get more than about 5000 characters per POI!