It depends what value of your company you can sell. If you can convince people that your company is worth 1 million USD then you get 200k for 20% or something like that.
So check your numbers, check your market, be good in selling.
Or course investors will try to get a better deal but responsible investors know that the founders need to keep a significant state after seed round, definitely more than half.
see as well here (this is a german company builder), just click on equity story towards the end of the page: http://www.rheingau-founders.com/working-with-us/
this all differs much depending on country and business but gives you an indication.
when to start seed round.
You made pre evaluation, interviewed potential customers and did some landingpage tests or similar things to get an idea that your business actually has good chance to get traction, then get seed to build a prototype with all these arguments why this investment is the best thing they can go for.