How many 34oz bottles would I need for a 6m bell tent?

1 bottle should be plenty. I Did a 10 man tent with 1 bottle

Does the 34 oz come with a spray on applicator?

Why don't people read the descriptions? It says it's a wide mouth to facilitate pouring into the smaller containers to refill the sprayer jars. It would be a little heavy and hard for your hand to grip it. Awkward to handle but a spray hose similar to those found on bug sprays could be retrofitted to work, just drill a hole the size of the hose to fit thru. Remember to clean it out between uses, or it will clog the hose and sprayer. Likewise, it does a great job of cleaning my gear after each outing.

Is this applied to outside or inside of tent rain shield? Is it necessary to also use seam sealant?

Are the seams leaking, or is the original seam sealant 9ld, peeling and/or cracking? If so, reapply seam sealant. Use the Nikwax according to its instructions. You definitely want the outside to be clean, waterproof and solar proof. If there are dirty spots inside, trry spot cleaning them as well.

How much solution will this one bottle make?

The instructions say 2 parts of the product should be mixed with 5 parts of water. The container contains 1 liter, so the bottle should be mixed with 2.5 liters of water to total 3.5 liters diluted solution.