The ownership target is dependent on the manager's thesis and their model. For example, at the pre-seed stage, it's very common to see a higher volume of investments to compensate for the risk. However, the average valuations of the investments will vary widley by the sector and region. For example, valuations are lower in emerging eco-systems vs established ones.
I recommend you work backwards here. Start with understanding the average valuation in your eco-system. Then, review the exit values of previous startups that are on thesis. Next do some back of the napkin math to figure out what type of exit you would need for your average ticket to return 1-2x the entire fund size. Finally, use all of this information to figure out your ownership target. This approach will give you a good start. Then, you can take this to the next level by plugging into a more robust fund model like this one: https://foresight.is/vclab/model