That would I believe be a notification coming from NRaas StoryProgression. Most of my worlds are way too tight on housing. Once I start playing a world for a while, I always let more sims in than I really should by way of SP immigration or just by taking a liking to them when they are homeless NPCs or friends made in foreign worlds and move them in as residents myself. And then, whether I am playing them myself or SP is in control, I let them have more kids than I should. Eventually of course the kids grow up and either partner up or just want to move out on their own. And of course along the way other households will want to move into more expensive homes or those that better meet their needs if they are failing housing inspections (wealth moves), or might need to downsize if they are living beyond their means. And some households will want to split apart for various reasons, others will want to come together and live with friends. These are all products of a healthy story progression trying to manage things.
But with no emigration allowed, as it's off by default, generally worlds do become crowded as the only way sims can leave your world is to die of old age or of other causes.
To help get a handle on what your towns' sims are nagging you for, on City Hall run NRaas > MC > Demographics > Lots > "X" to dismiss the filter. This should tell you how many residential lots you have and how many are unoccupied. It will also tell you how may are entirely empty, but of course sims cannot move onto empty lots on their own. Assuming you have any unoccupied houses at all, then go to Edit Town and see what price ranges the available ones are in. You might just have available housing a bit out of balance by cost for what your town needs or you might have none at all and everyone is getting stuck in overstuffed houses since they have nowhere to move out to or nowhere that they can afford.
To see how many sims and households the mod or you are trying to accommodate, again on City Hall NRaas > MC > Demographics > Population > "X" to dismiss the filter and look for the number of Residents. Resident Households shows up on one or the other of these reports, sorry am not in-game at the moment and I often forget which.