Do you need more yogurt starter for more milk?
Recipes call for a certain amount of starter to maximize the chances that your starter bacteria will crowd out undesirable wild bacteria.

If you use too little starter you will increase the chances that some random bacteria will win the incubation war. Since you don't know what you will get this can be actually dangerous.

I would recommend making an initial batch with the ratio the starter recommends and then using that finished yogurt as a starter for a second, larger batch. Since you would then have the entire first batch to use as a starter you can scale up your recipe for larger batches.