There's nothing that explicitly makes taking on more units in your first deal a *bad* idea, exactly, just one worthy of a great deal more caution. As you mention, anything over four units is going to be considered commercial from both a lending/mortgage perspective and an insurance perspective. Commercial financing is a ton more strict, and generally the repayment term is quite a lot shorter. I'm generalizing because plenty of people have certainly come up with creative ways to get commercial property financed, but overall, the terms aren't as favorable as residential lending.
The bigger concern for me would be the potential for being undercapitalized-- eight units means potentially eight or more each of the items that wear out, get broken, etc. If you take on a SFH or duplex and find that your initial repair budget gets blown, it's probably blown at a much more manageable level than it would be if you purchase an apartment building and require unexpected major repairs soon after closing.
For example, I closed on a duplex in January. I had the property inspected and I had my two different contractors in for rehab quotes. I was pretty confident that my up-front repair budget would be around $5,000. In the months since, I've gone through a few major repairs, an expensive eviction of an inherited tenant, and some "bite the bullet and get it done for the long term" repair/improvements that drove that cost up to close to $12,000. I've got the money, but if the same kind of thing happened to me if I was expecting $20-30K in up-front repairs, I would have to raid other investments to cover the costs, and that would be no fun at all. Someone with a smaller stash than me might end up with a repair bill that they couldn't cover at all.
Don't get me wrong, I actually love multifamily properties for a lot of reasons and would love to own a small apartment building some day... but I've shied away from it so far because an unexpectedly expensive repair could jeopardize my long term FIRE plans.
