Would getting an NIE number help?

Please don't take this as gospel, but they way I read it (at the moment) you would have to at least apply for residencia before 31st December. 

Hopefully someone with a lot more knowledge than me will be along to give concrete advise.

An NIE gives you no rights to residency. The NIE is simply a foreigner's identification number which is needed for various financial transactions such as taking out a mobile contract, opening a bank account or buying a house. Once you have it, it is yours for life.

You don't have to buy a property you can take out a long term rental instead but it does have to be your main residence.

Ok thanks.

We’re just looking to buy a holiday home anyway, so it looks like there’s no rush for the NIE. Would we need to apply for Residencia if we only plan to visit a few times per year?

Once we retire, we plan to spend 3 months, twice per year there.

Post Brexit, Dec 31st, you will only be able to spend 90 out of 180 days here. The clock will start once you enter the Schengen zone so if you plan on driving you will have to count from the minute you set foot on Schengen soil. You could be here from Jan 1st for 90 days, then away for 90 days and so on. It will include any other holiday you may take within the zone.
You only have to apply for Residencia if you plan to make Spain your main home. This would entail becoming tax resident here and changing your UK driving licence to a Spanish one etc.
For a thorough grounding, read Jim's Guides which can be found at the top of the page.

I am on the understanding each 90 days visits In each 180 days per year is allowed at this present time as a non resident, what happens after brexit 31st December 2020 unfortunately I don’t know.

So am I correct in saying that there’s no benefit to getting an NIE number before 31st December if we’re just planning on buying a holiday home?

Passports for UK residents have not been registered on arrival or departure but they will be from 1/1/2021.

Yes.

That's right, unless you plan on buying you holiday home before 31st December.