You need to build a new lookalike audience to increase market share for a GCP...

Choose an audience size close to one to optimize for similarity.

Since this is around 1-5% of the total population of the country you are targeting, your Lookalike Audience is likely to be small, with a high resemblance to your seed.

If you are optimizing for reach, you'll want to choose a number near 10

This Lookalike Audience will be larger, but less similar to your seed, since you're targeting up to 10% of the total population.

This example would create 3 Lookalike Audiences:

0%-1% of the population, which will be the most similar audience to your seed.

1%-2% of the population, which is second most similar, but a bit wider reach.

2%-5% of the population, which is the least similar but the largest reach