AFAIK LW Basic allows a maximum of 3 UP's to connect thru & LW Pro 5. However if you run as an Ultrapeer then you'll get more.
No it doesn't necessarily help having more than a certain no. & what it does is tie up potential connection options for others trying to connect on the Gnutella network. 3 works fine with LW & 5 for Pro. At times you don't even need that many if you're not in front of your computer & searching.