How does company's share price get affected by promoter's stack percentage?

Promoters in any business establishment hold more shares. They are the one who know exactly where the company is heading to. It would therefore be a wise decision to track what these promoters are doing with their shares.

Any promoter going for an increase in their stake can be a good sign for the well being of a company. A promoter investing in a company would indicate that he is confident of the company's prospects. However, there are some school of thoughts that think otherwise as well. It would be practical to understand the purpose of hiking the stake. A promoter may also buy more stakes if the company is planning a delisting. It can also happen when there is a likelihood of a corporate restructuring. A hike in promoter stake alone cannot be treated as the sole reason for buying a particular stock.
In essence, though it may indicate better prospects, you should not treat it in isolation and rush to buy the stocks just because there is a hike in promote stake. Pay attention to the regular aspects of stock market trading and a normal valuation of the company from other perspectives as you would normally do.

Let me answer your two questions separately.

Impact of Promoter on Share

For any company to grow, the promoters are very important component for their business. And the investment the promoters do in their company decides how much growth potential a company has in future. If the promoter is investing the right way, it can affect the shares and benefits the company. And vice versa can be true as well.

If the real owners themselves are promoters then their every action decides how much potential a company has over a certain period. If they are selling the assets then the market gets the idea that company and it's share are not going to be profitable. So the role of promoter as an owner is also very important.

If the promoter is in board of directors or the brand, then that also affects the holding if they are not investing more. So in such case the scenario goes against the shares, if the percentage of holding turned out to be lower.

In short, the promoter is the face of the company both directly or indirectly, their holding, action in board and the performance decide the company future value. This in turn affects the share value in the market. As the people invest for brands while tracking the promoter percentage, the promoter acts like the captain of the ship. If he abandons the ship, then it does affect the company and its shares.

Which stock to buy if the promoter holding is more or less

If the promoter has less holding in the company, then no matter which brand it is, it is better not to buy that brand shares for long term. You can keep it for short term and sell it. If you see the promoter holding is more than 75 or more, then you can invest into such stock for longer terms. Regarding, which stock to buy and which not, specifically, that sort of prediction would be like making too much assumptions for this answer (or for that matter any answer).