I cannot answer that question. There are too many variables for anyone to give an accurate answer.
Is immigration necessary? I can answer that question. Yes, it is. How many people we allow to enter the country is not the question we should be asking. We need to ask, "Why is immigration necessary?"
One reason of course is our aging population. More and more people are living longer and longer. That is true everywhere. My godfather died just last week at the age of a hundred (and five months!) He was an old man, a man who should not have lived as long as he did given his war service and the injuries he sustained then. Excellent medical care helped as did a caring family.
Even people with no family to care for them are living longer but they often need extra help, perhaps to stay at home or to live in a facility for older people. It means an increase in things like age-related dementia and the need to care for those who acquire it.
We need help with the delivery of all sorts of services and skills like building housing. Those things are very obvious.
We do not need more yoga teachers however much we might argue that yoga is good for our physical and mental health. We do not need hairdressers or manicurists or any other similar skills.
We also do not need people who are not going to integrate into our society and accept our laws, our Constitution, or our culture. It is time to stop thinking about "multi-cultural" affairs and think about a cohesive society. This in no way should stop someone retaining some aspects of their birth culture inside their own homes, aspects which do not break the law of this country. It should not prevent us from enjoying a greater variety of cuisines or a range of other cultural activities. Such things add to the capacity of everyone to accept one another. The idea that these things should in any way take priority over the laws and Constitution or the foundations on which this country was first built is wrong.
What we do not need are laws and policies which encourage or even prevent people from integrating into society. This happens all too often.
We are also far too ready to accept people into this country and then expect them to work at the low level jobs that those who were born here do not want to undertake. Surely we should be asking questions when a plumber who has qualified in England cannot get a job as a plumber here? Why does a doctor who qualified in Scotland choose not to come? Is it because of all the difficulties put in his way? Why is an engineer from Taiwan (with excellent English) driving a taxi? There are plenty such examples to be had and it is time to do something about it. We need to recognise and acknowledge their skills and do so in a timely fashion.
We also need to insist that at least some of the unemployed people here are not able to simply appear to seek work. They need to actually seek work and accept work which is offered to them. They need to actually attend and finish training courses. Many do of course but those who fail to attend and finish training courses and then accept jobs which are offered them are making it much harder for everyone. It also means increasing immigration in ways which may not be best for society.
Any "multicultural" approach must also recognise that there is no single "indigenous culture" and no single "indigenous language". What was there when white settlers arrived was a very diverse range of cultures and languages. What often remains now are no more than remnants and even those are often so tainted by interaction with others they would be unrecognisable to those who lived them two hundred and fifty years ago. Yes, it is hard to realise how much we have lost but what are we actually trying to retain?
Claims that we are some sort of "highly successful multicultural society" are something we need to which we need to give much more thought. It might be that it is the very thing which is causing division rather than cohesion.