while launching an ICO , usually how are the token prices decided?

You should have posted this topic in Altcoin Discussion as we are discussing about ICO. A token price during an ICO is set by the founder of the coin/token. Most of the token founders set their token price as 0.1$/token so that more people would invest in them considering the coin/token price. You need to understand that a ICO and a market price are different.

Most of the tokens/coins open up in the exchanges less than the ICO price. This price is called as the Market price. If there is a higher supply of coin/token for example it is around 1 Billion the price would be low unless there is a huge demand for the coin in the market. If the coin supply is too low and there is a very huge demand of coin in the market, the coin price would shoot up too high such as Btc/Ethereum. Btc has a very limited amount of 21 million coins which would be generated and being the first cryptocurrency has a high demand for it, hence the rise in price.

Coming to your second question, the share hold down by the company is based on their own wish. You could find the % of total supply retained by the founders in their whitepaper. There is no such amount for them to hold.