To illustrate, this reads a bit awkward:
In a country with 75 billion SMBs, at least seven billion of them should advertise.
What's the acceptable practice in such sentences ?
The writing style I have come across most often is to write out in words numbers of ten or less (occasionally twelve or less) and use digits for all higher numbers.
In a sentence, using words gives a better flow, but obviously with higher numbers that flow would break down anyway, so I think your sentence works just fine as is.