The writeup Meraki provides as to why no more than 3 SSIDs are recommended is excellent. For a TLDR explanation, basically each SSID being broadcast has to send out a beacon, so that client devices can find it. The more beacons, the more congested the airtime - which especially on the 2.4ghz band, is precious to begin with.
For us, we generally operate a "Public/Guest" network, an internal employee network, and a third that varies depending on the location. Sometimes in our restaurants, we have a specific network for the restaurant to attract non in-house guests (as we operate hotels). We also usually have a "Group/Meeting Space" network for our ballrooms and large group areas, that often has different bandwidth allocations and/or a lack of a splash page.
We have scaled up to 5 SSIDs on a few very busy APs, and haven't noticed any huge drawbacks - but your mileage may vary depending on your RF environment.