Depends vastly on subject and type of Uni.
Places like LSE, KCL, UCL etc get swamped with applications for subjects like Economics, not because they are 'better' courses but because of the supposed glamour of being in London. They cannot make an offer to everyone who 'meets' and this means that they will reject folk who do actually meet the A level entry requirements but have been weeded out because of overall GCSE profile, PS etc.
Every top Uni will have its 'mega competitive courses' - it'll be different for each one. But even top Unis will have 'cinderella' courses that are hard to fill where they essentially make an offer to anyone who is even remotely close to the entry requirements.
This is why its always recommended that for 'mega competitive' courses/Unis that you only have one or two of those as a 'risky' choice, even if you do meet or exceed the entry requirements, and that you have other less competitive choices alongside where you have far more chance of actually getting an offer - typically 'out of London' or 'further North'.
Obviously there are plenty of Unis and courses that are not this competitive - the majority in fact - and for these, usually if your predicted grades meet the entry requirements you will get an offer. So, with one 'risky' choice, three 'meet my grades' choices, and one 'below my grades' choice, you have a good chance of getting 3 or 4 achievable offers.
Hope that helps. Ask more specific questions about your situation of you need to.