I have not heard that data sets like this are published. But at the ECSS conference in Dublin, the winner of the Young investigator award presented exactly what you are searching for. Perhaps you could ask if he could make his data available for you?
I do not know any artcile about exact microelectrode recording from many neurons. But I know a papr with multipatch recording from its that was performed by one group from the Stanford University if I´m not making some mistakes. And to me that was one amazing work. The Idea of study was to show how generation of the electrical signal in one neuron have some influence by electrical field on another one. This study was published in some journal of Nature group and it was about 7 years ago. I`´m not sure that I understand well your question but want to say that microelectrode recording is more easier than patch.
Using Multiple Whole-Cell Recordings to Study Spike-Timing-Dependent Plasticity in Acute Neocortical Slices June 2016 DOI: · 10.1101/pdb.prot091306 Txomin LalanneTxomin Lalanne · Therése AbrahamssonTherése Abrahamsson · Per Jesper SjöströmPer Jesper Sjöström
Are you looking for e.g., microelectrode array data from primary neuronal cultures? Do you need a raw data which contains all the signal information including background noise, field potentials and certainly if exist neuronal spikes? or only spike time points (again if any detectable extracellular action potentials)?
Do you need to know the actual number of neurons recorded by each electrode? That's a more complex issue.
Why do you need the data? I have some recordings which I used in my previous studies. You can check my previous papers and have a general idea on the data.