
Talk to your local dog warden, they will have the specific info for your council. Or Environmental Health, if you have no DW.
Generally speaking, barking is considered a nuisance if it is excessive and here at least, that is quite a considerable amount. We're talking 20 minutes or more of literally constant barking - in some places, if the dog pauses for a few seconds then the clock starts again.
Often the complaints are made up by vindictive neighbours but sometimes they do highlight an issue - mine did. 5 of mine had SA and I didn't know (always had housemates until that point so they were suddenly alone) and of course, they were barking and howling when I wasn't there, so I couldn't hear them. I set up my phone to record after I received the letter and discovered the extent of the issue.
That said, even if it wasn't happening, I'd most likely have had a letter anyway because my neighbour at the time was just nasty. Apparently there are dozens of complaints on my file at the council about me and the dogs, and they will all be from the woman. None ever went beyond her calling them because there was no actual complaint to be made. She just hated me. She frequently rollocked me for the dogs barking '24/7' even after the letter, when I had someone in with them every single time they were left until I'd worked on the SA to the point of being able to leave them alone.
My point is, don't panic! Set up a recording device. If there is an issue, then get a plan in place and tell EH/dog warden what you're doing, as that will help if it goes any further, because it shows you're taking steps to address the noise. If there isn't an issue, then forget about it - most complaints don't get further than the diary keeping because at that point, the complainants either realise that their lie won't be taken seriously without evidence, or they try and continue, get noise recording equipment put in and then of course, nothing is found. You'll probably not hear any further than the letter you've had, if that's the case.