I know some engineers that can do all the tasks above, but not all of them with equal levels of skill. For example, a developer can probably design a web page, but it's not going to look anything close to Airbnb
So when thinking about staffing and how many FTE's are needed, the matrix of skills necessary for a digital product comes into play and becomes a factor on sourcing the right team members. The Silicon Valley model is to try to find individuals that are "full stack" — designers that can code HTML/CSS; and engineers that can code frontend, backend web, mobile and do QA and devops. The more full-stack, high quality, and productive a resource is, the higher the compensation the individual can command.
This skills matrix (and level of skill inside this matrix) is an area of ruin for business people that seek to save money on salaries. If the execs in New York decide to hire a $75,000 developer, they may actually need 2 more hires to produce at the same level of one $150,000 developer.
The skills matrix is also challenging when considering small teams and small budgets. In my budget scenarios above, I referenced a team of 2 FTEs. However, it is difficult to create an advanced, fully featured digital platform, product or service with two people. The recent pressure to support web plus native mobile platforms has generally increased the team sizes needed to be competitive.
This is a gross generalization, but I'll take a stab regardless. The following table generalizes the design and engineering FTEs needed for different types of digital products in the initial early stages of their life. And since we have this table, we might as well fill in the budget (with a very wide cost range to cover your team sourcing options).