Not an expert here, but there probably are many other strains out there with fewer mutations (and some with more). Most of them residual because they don't provide a clear advantage. The reason we know about this 17-mutation one is that this specific one turned out to have such an advantageous infectivity that it's displacing all other strains and becoming the dominant one.

Thanks.

That would make sense, if it was the 17th mutation that (possibly together with the rest of the mutations) gave it this advantage.

The problem is most countries don't sample - they put people through a PCR test and then send them off their way. The UK also does somewhat-regular genetic sequencing of positive tests, precisely to spot new mutations.