I want to make 30A MCB but in market 1,2,4,8,10,12,16,20,32 A MCB are available. Can I add as 20+10 or 3x10 in parallel to make custom 30A MCB. Or I should replace 30A with 32A without hesitation.
I assume an MCB is a type of breaker:
In that case, if you need 30 A, you'll need 30 A.
Putting breakers in parallel is a) most definitely against any code and b) will lead to surprising behaviour. Especially with reactive loads (I take that's why you have motor in your question), sequentially firing breakers is a big no-go.
You also can't just generally substitute 32 A for 30 A. If you need 30 A, you need 30 A, though I'm having a hard time imagining an application where that's the case. But you're the one who set the requirements, and these seem to be 30 A, not 32 A.
MCB stands for Miniature Circuit Breaker, which are rail-mount circuit breaker devices made to IEC standards for use in circuit conductor protection. The sizes you are seeing offered are based on standard IEC conductor sizes, so the reason you see "32" amps instead of 30A is because the metric conductor size this corresponds to is capable of carrying 32A. In North America we basically do the same thing, so for us we have a 30A breaker because it is generally intended to protect 10ga wire, which is rated for 30A.
NO, you cannot "create your own" rating by combining breakers in any way. So you either have a circuit conductor rated for 32A, in which case you use a 32A breaker, or you are in North America where you are using 10ga wire. If the later, you can't use IEC MCBs as Branch Circuit Protection here anyway, so the point is moot. You have to use a circuit breaker that is listed under UL-489 and if you look for that, there are some MCB manufacturers who sell versions of their MCBs that are indeed UL-489 listed, and they will usually then offer a version that will be rated for 30A, not 32A. These will not be the cheap stuff you find on the internet auction sites however, they will only be available be from larger more reputable suppliers.
There is a reason why the circuit breakers don't come in any value. The same as for resistors , capacitors etc.
They have a tolerance. There is no point in having 32 and 30 A breaker when the tolerance is somewhere around 30%. A good 20A breaker can have a real value of 18 or 25 due the temperature or other reasons to fire anywhere between 18 and 25.
So just use the upper closest value for your MCB.