Details regarding the application are found here:

The paper form posted on the above is aimed at residents of the Channel Islands, Isle of Man or a British Overseas Territory not served by UKVCAS/Sopra Steria.

People living in the UK should apply online: AN form
The guide is here: AN Guide Naturalisation Booklet

Photo Guidance.

The photos are affixed to the referee declarations.

There is no cap or quota regarding the number of citizenships granted.

See: Step-by-step citizenship application