How to handle the situation when you unable to perform @ work.?

So best case scenario, you deliver your 5 projects in 15 months and your competition will be claiming they could do the same work in 3. Your teams estimating maturity will suffer because they will only be able to consider 20% of their available labor. You may find that you actually are unable to perform some tasks in a single sprint.

If you have to change the number of projects being worked from 5, your team will have to adjust their estimating habits which will damage the teams efficiency. Additionally, your team will find it difficult to self-organize when a simple task reassignment may require spinning up a new dev environment before work can begin. If you were to run the same 5 projects serially, you'd deliver the 5th project in the same 15 months, but you would have educated your client that your team is in such demand that you have a 12 month backlog and that you can use that time to refine your project goals.

Or if you have a constant backlog, you know it's time to start hiring another team.