We can use as many as you want. But it is better to use maximum 3 keywords per page. For this you need to divide your page into subpages with each one taking its own topic.
A professional SEO expert has to study the latest updating of search engines by taking 2 to 3 hours every day. Without updated techniques a SEO guy is a big zero so how can a web developer can do SEO? A good web developer can support with attractive design only.
Search engine needs minutes to weeks depending on the website to index it or new contents appearing on it. But listing for the keywords will be based on the competition for the keyword and number.
It will take a while depending on what results you expect what for. Some keywords might even take a year or more while others may work after weeks or months. Perform a thorough keyword and market research first to know. Also SEO is basically a continuous process that never stops.
SEO is a full time job for a person or requires a whole team with several skills ranging from web development, content creation to marketing. Unless you have at least one experienced full time SEO in house it is strongly advised to seek assistance from a SEO company or consultant.
It takes lots of time and need to do lots of researches, studies, quality link building, content development etc. for getting better positions for best keywords.