Need advice ono a Tube preamp

If I buy a tube preamp new

Why new versus used?

then:
1) how long does it take to burn in

That depends on the preamp...and the caps, resistors, etc. But, in general 200-300 hours.

2) how often do you need to change tubes

Depends on the tubes. I am only familiar with 6922 signal tubes. Excellent quality 6922 tubes made in the early to mid 1960s by Amperex or Siemens will last 3000-5000 hours or more. Premium quality 6922 PQ or 7308 tubes by the same companies were designed to last 10,000 hours

3)will changing the type of tubes affect the sound much/little and if so are there certain tubes that are considered to be better than others?

Again, this depends on the tubes. The short answer is yes, changing tubes will affect the sound. I suggest reading Joe's Tube Lore for a comprehensive description of signal tubes and their sonic characteristics. Although this is Joe's opinion, many have found his descriptions to be quite accurate. I have not discovered any other information source to equal Joe's Tube Lore for signal tube guidance.

Have fun. Consider buying a used preamp with an excellent reputation made by a company with first rate customer service.

TAD-150. Read the reviews...

I see some tube preamps have one tube,some 2 and some 4.I only need a line stage preamp.I would also prefer balanced outputs.As I am trying to warm up the mids and highs a bit is it better to have more tubes? Or does the number of tubes even matter?

You can accomplish what you want with a tube preamp. However, rather than focusing on the number of tubes a preamp has I would rate your priorities as follows:

1. Design, including how the tubes are used in the signal path and/or power supply.
2. The type of tubes being used. Each tube has certain characteristics that "color" the sound. Look for one that has the characteristics that match your sound preference.
3. Abililty to tube roll. This is not so important to some, but to me having this flexibility is a nice-to-have feature. You'll want to know what NOS options or alternative tube types will work with the preamp.

A lot of people are hot on the Modwright preamp right now. It might be worth a loook. Me, I'm biased, but I'd recommend you look into a Joule Electra LA-100 MkIII. Here is one listed on Audiogon:

http://cls.audiogon.com/cgi-bin/cls.pl?preatube&1138632048