How to choose an agency and how many agencies to use
Search engine optimization (SEO) is the process of improving the visibility of a website or a web page in a search engine’s “natural or un-paid (“organic” or “algorithmic”), search results.[1]
Before moving forward, ask yourself a few questions which will help you to decide:
1. How many leads or sales per month/week/day do you want to generate?
2. What is your estimated budget?
3. Do you have different verticals/domains and whether the keyword set is vast or limited?
If your web resource is small and aimed at one specific type of business, then one agency is enough, otherwise there are just too many over-lapping aspects that could cancel each other out. The agencies would be competing against each other and it could hurt your overall SEO campaign and its rating. Choose a company that has a good understanding of the keyword search volume. So it’s better to choose an agency with a good reputation. Keep in mind that it could be costly.
If your website is a big portal and has different departments, then you can hire 3-4 agencies based on their previous work. These agencies would work on different sections of your website, creating different keywords for each section. In this case their independent work will not be negatively affected by the work of the other agencies. Using a multi-agency approach will enable you to choose based on each agency’s performance.
If you do not have a big budget or you’re looking for a simple promotion campaign for your small business or start-up, do not waste your time on SEO which was created by Google for Google searches. Instead, create simple user-friendly profiles for your company on Glabex.com; list your all products, upload product images, and upload videos. This service is free for any business, and it allows you to create an online presence for your business.
[1] Wikipedia
selleasy 
Don’t forget to include the keywords in your copy! If you use keywords in your title, description, alt tags, etc. that don’t appear in the copy, it is unlikely they will affectse your Page Rank. Use the keywords in the copy as well, but make sure they’re included in a way that reads naturally.
Thank you, Ash for your advice.