There absolutely aren't 'only so many jobs'.
If you think the number of jobs and the amount of money in the UK are fixed, then the purpose and/or consequences of almost all government policies will go completely over your head.
You can think of it as spirals, which can spiral up or spiral down:
Increasing jobs and wealth spiral through keeping experienced workers:
People with high levels of capability and experience keep working in the uk (not retiring or moving abroad) -> business chooses right profit-increasing initiatives and implements them successfully -> business makes more profit (and pays more tax) -> business opens more of it's operation in UK / invests more money in production-growing infrastructure / employs more people -> high value jobs are available for people with high levels of capability and experience, pays them well and makes them feel appreciated -> People with high levels of capability and experience keep working in the uk (not retiring or moving abroad) -> and continue the spiral above
Decreasing jobs and wealth spiral through losing experienced workers:
People with high levels of capability and experience retire/reduce hours or move abroad -> business can implement fewer initiatives or initiatives fail through lack of capability/expertise -> business makes less profit (and pays less tax) -> business viability goes down -> business closes operations in UK / holds back on investment / reduces or doesn't add jobs -> fewer high value jobs are available for people with high levels of capability and experience, or pays them less -> People with high levels of capability and experience retire/reduce hours or move abroad -> and continue the spiral above
And these spirals play out for all sorts of other choices. People who shout for businesses to pay more tax with seemingly no understanding of the consequences (eg Angela Rayner) should be more aware of these spirals:
Increasing jobs and wealth spiral for business viability:
UK people work more and create more value -> economy grows -> businesses sell more and government tax regime gives them confidence in the viability of the business -> business opens more of it's operation in UK / invests more money in production-growing infrastructure / employs more people -> UK people work more and create more value -> and continue the spiral above
Decreasing jobs and wealth spiral for business viability:
economy weakens -> poor government policy may consider increasing business tax and regulation 'to help people' (hello, Labour!) -> businesses sell less due to worsening economy and also make less profit due to increased tax and regime -> business viability goes down -> business closes operations in UK / holds back on investment / reduces or doesn't add jobs -> UK people work less and create less value -> economy weakens -> and continue the spiral above
I actually think the fundamental difference between Labour and Conservative policy is that the Conservatives concentrate pretty hard on getting onto the 'Increasing jobs and wealth' spirals and avoiding the 'Decreasing jobs and wealth' spirals... whereas Labour behave as though they don't exist.