To answer this question I will be referring to individual (classic) extensions as this is what I specialise in and teach.
When applying lash extensions you should be aiming to cover at least 90% of healthy natural lashes.
When you're just starting out you probably aren't getting that many, but try not to put too much pressure on yourself - I know its easier said than done, but I promise it will come with practise.
The picture to the left is my first ever set of lashes, where I thought I covered 100% of her lashes - I actually covered around half! The right photo is the same person about 2 years later, where I actually covered over 90% of her healthy lashes.
Its really important to remember that even when you lash every healthy lash, how full the finished set *looks* will depend on a few factors, and most of those you can't control. This is where managing clients expectations is essential:
1. How thick/healthy/dense the clients natural lashes are is the single biggest factor that will affect how full they look at the end. If your client has sparse lashes to begin with, then the finished set won't look super fluffy and full.
You need to work out what she is expecting and make sure she knows what the end result is likely to look like, and what the limitations will be.
2. The lash extensions you use will affect the finished 'fullness', but not much. In general, flat/ellipse lash extensions will look slightly fuller, and you will be able to up the diameter because this lash is lighter than classic round extensions. Matte finish lashes may also look darker than the glossy finish lash extensions. Of course a wider diameter of lash will look fuller too, but this will depend on what weight the natural lashes can safely take. (Remember that .20 is the heaviest lash you can safely use for classic lash extensions)
3. If you want the finished set to look fuller, try going for a shorter length. This might sound a bit backwards, but if you keep your set shorter, the lash line at the tips will look less sparse and wispy, and will result in an overall denser looking set. Its essentially a trick of the eye, but its worth trying if your client has more weak or short lashes than strong healthy ones.
Just one last tip to fill out your sets - don't be afraid to lash the little ones with a 7 or 8mm extension - as long as it can take the weight. This will darken the lash line at the root, and as they grow over the next week or two, they will catch up to the others and fill out the lashes as the longer ones fall out.