A brand new product may start out with very few contributors. It's probable that a singlue user owners most or all of the vested coins. In that case, the user has wide discretion in how to allocate further coins to subsequent contributors. Several questions arise, such as:
See our section on bounty valuations
It's up to you! But typically, a brand new product has very little value; you'll have to offer a lot of coins to incentivize new participants. Conversely, an established, profitable company might have a lot of value. Then new entrants may not be able to expect a huge ownership stake. Try to gauge the amount of work that has been put into the product. If it's a brand new product, it might be smart to be generous to new participants. Owning 100% of zero is still zero!
Probably not. Building something really awesome will involve working with others. You should try to be fair with coworkers. Remember that giving them ownership incentivizes them also to see the product succeed. Being too miserly means you have no one aligned with the same incentivizes.
Also the greatest risk any product has is obscurity. You are far more likely to miss out on value from a product that fails, than from having given out too much ownership. Bringing in talented people is essential and almost certainly worth the cost.
When migrating an existing product onto LET´s CREATE, you can choose to allocate to yourself any number of coins out of the original 10 million. Since the product was yours to begin with, it's totally up to you to determine how many coins should be vested (in your hands) versus unvested (available for payouts for new contributors).
Generally speaking, leaving a large portion of unvested coins might be wise. Unvested coins need not be spent; you are under no pressure to award them at a pace you are not comfortable with. Having a large number of unvested coins available gives you breathing room to compensate people for work done. It also allows you to bring in the best people. While leaving more coins unvested increases the upper bound of dilution that you could experience, it also expands the scope of better utilizing Assembly. After all, it's all about working with other people; having more unvested coins available facilitates that.