Improvements and next sprint

There's no singular answer to this question.

I would say the minimum is one. You want to be making continuous improvements to your way of working. If you haven't identified one potential improvement and set some time aside to begin to work toward implementing it, you aren't practicing continuous improvement.

The maximum number depends on the capacity of the team. Like any kind of work, implementing improvements takes effort. The more effort you dedicate to implementing improvements, the less you'll have for product delivery work - you'll need to balance delivering value to your stakeholders with improving the team's ability to deliver value.

I'm starting with SCRUM and I want to know how many improvements of the retrospective should be included in the next sprint ?

In lean and agile practice, the best way to inspect and adapt is as closely as possible to the time and place of work being carried out. So ideally the answer would be none: the fewer that are delayed to the next Sprint the better.

As Thomas mentions, there is no best answer for this. He has some great points to consider. We want to ensure that some improvement is identified and made.

I like Ian's view of this. To quote the guide "A Scrum Team is expected to adapt the moment it learns anything new through inspection."

Some other thoughts on this...Just because we have a formal event for retrospection, it doesn't mean it is the only time we inspect and adapt ourselves as a team. Why wait for the end of the Sprint to Inspect and plan Adaptation when you can improve as you go. Our improvements are not limited to formally planned activities in the next Sprint. Of course they can be, and if retrospective is where something is made transparent, that would be when you address it or plan for it. 

Agree with the points mentioned above. There is no clear answer for this. It is what the team decides and finds suitable. There is a balance you need to strike between improvements and new features (there is always a push for new features :) ). Sometimes improvements could also be a technical debt incurred (some adjustment done to prioritize time over quality). These technical debts needs to be cleared immediately as if it is put to a later date, it would mean a big effort later.