Team Velocity changes if backend team not involved?

How many points we should take? 

None yet, by the sound of it. From what you describe, the team would be unable to commit to creating a Done increment. It apparently has sub-teams and therefore lacks the integrity to do so. That's the problem to be solved.

At this point, it doesn't seem like you have a Scrum Team:

The fundamental unit of Scrum is a small team of people, a Scrum Team. The Scrum Team consists of one Scrum Master, one Product Owner, and Developers. Within a Scrum Team, there are no sub-teams or hierarchies.

It does seem like you have a cross-functional team, in that the team, as a whole, has all of the skills necessary to create a valuable Increment every Sprint. However, the individuals are not only highly specialized, but have formed sub-teams that are not fully focused on the singular Product Goal or Sprint Goal.

There's no reliable mathematical function to tell you what your velocity will be. However, I can say a few things with a high degree of confidence. Someone working outside of their comfort zone will take longer to do a piece of work. Similarly, someone who is learning the tools and technology will also take longer to do a piece of work. If you ask someone to teach others, they will also have to work slower.

If you do what I would consider to be the right thing and have your front-end team train the other members of the Scrum Team on good, effective front-end development, your short-term capacity would likely be less than the capacity of just the front-end team. However, you would be giving more people with front-end development skills that, in the future, can take on front-end development work and do a high quality job with it, increasing the ability of the team to deliver.

If you do what I would consider the wrong thing, you would have a piece of your team sit idle (or work on entirely unrelated work, not focusing on the Sprint Goal) and you would not be growing a team that is in a position for long-term success and value delivery. However, your front-end developers may be able to get more work done. How much more is unclear.

Agreeing with everything that @Ian Mitchell and @Thomas Owens have said.  The real problem you need to solve is how a team has gotten to the place where there are sub-teams. 

But for sake of argument, let's assume that isn't a problem because you have a team that is capable of doing all the work that is needed in order to complete the Product Backlog Items.  The problem I see is that the team has gotten into a habit of refining work into silos of knowledge rather than in delivering value.  How did they get to a point in time where all of the work that is needed is entirely front-end?  That indicates horizontal slicing work instead of vertical.  Are you really delivering value to the stakeholders every sprint?

Your question actually causes more questions in my head than answers.  

I agree with above comments. Additionally,we should think how effective retro is? Are these points being discussed? Also, has team spent time in defining DoD and agreed what they call really done. Thanks Ankush

Hi, Saher!

Even if, as you issue, your team is cross-functional:

1. Do the items in your backlog represent features (for instance, User Stories) or do they represent components (for instance Technical Tasks)?

2. Do you have a Sprint Goal on each iteration? Does the Sprint Goal represent a full increment?

3. If you are working on technical tasks instead of features, is your team able to deliver usable and releasable increments on every the sprint?

Thank you all for your reply

@Gonzalo 

I always get this question during interviews and I reply back exactly with the same inquiries that you have mentioned. But they always reject me after I start to ask as if they are waiting for a certain answer or as if there a model answer for this question.

So I got confused and I am not sure how should I answer such a question if I didn't get the info that I need to answer it, actually they refuse to answer my inquiries.