This is on purpose. You'll have to set the new number anyway so you can set the box too.

I dont have to set the number if I am exporting different timelapses at the same time. not batching the same one.

Yes, but if you are doing different time lapse sequences at the same time it makes no sense to prevent LRT from rendering, so it's good that the box is unchecked then.

You'll only have to check the box, if you split one time lapse in parts and work with the "Staring number" feature, in all other cases, just leave it off.

I've just LRTimelapse 4.3 that fixes the issues with simultaneous exports.