DS is nearly 8 and pretty good at maths - he has a good 'sense' of how the basic operations work, and gets place value, so he is pretty strong at mental arithmetics.
At age 4, before starting school, he could count (objects or actions such as steps) beyond 100, and recite numbers as far as you like to go, and match quantities to numerals. Find one more or one less, and because finding two less is just finding one less and then one less again, finding 2 less as well. But he didn't have much of a 'sense' of quantities and numbers, e.g. he didn't 'get' that 6 can be split into 4 and 2.
Some of the 'techniques' he has been taught at school have been more of a distraction than anything else, and all the memorisation (number bonds, times tables) - though easy for him - has taken away from developing more of a 'sense' of numbers and quantities. By instantly being able to say that 4x5 is 20, he doesn't get to practise his 'sense' of the four sets of five that are in 20. Since memorising his tables, when asked a multiplication question within the 1x1 - 12x12 range, it is the language/word/memory part of his brain that kicks in, rather than the maths/numbers/reasoning part. It is only when multiplying beyond 12x12 that he applies his 'sense' of multiplication and numbers. And therefore he stopped practising/firming his 'sense' for eg. the four groups of five that are in 20.
Similarly with number bonds, as he has always found memorisation easy and fun, he memorised number bonds to 10 at a young age. And because he 'gets' addition, has always been able to apply them too. But has stopped developing his 'sense' for how the numbers up to 10 fit together - too early IMO.
DD in contrast, is four now, and has a much greater 'sense' of numbers/quantities than DS had at that age. She will randomly (e.g. whilst cycling to nursery) announce that e.g. 7 can be made up of 3 and 4, or that 6 can be made up of 2 and 2 and 2. She claims she can count to 100 but in reality will stumble around 29 or 39, and confuses 12 with 20, but I would say her number sense is vastly better than DS' was at that age.
There are clearly multiple aspects to 'number sense' and sometimes you will develop the 'sense' after the technique/memorisation by lots of applying it, other times the 'trick' or technique will distract from the 'sense'.