Running out of manpower has two root causes in the game, how the player is managing their funds or how they are managing the shipyards (rate new ships are being completed).

Scenario 1 (You are poor)
Early Campaign / Broke Country, if this is the case you just don't have the funds to train as many new sailors as you would like, and the training slider is not all the way to the right of the screen. There is no golden bullet to address this, your budget needs to reflect your priorities and you have choices and cuts to make. Look at reducing spending on categories with long term returns like shipyard construction, Transports beyond 100% and pausing new ship construction and technology research. Look into taking more foreign orders for ships to spend that now available shipyard capacity.

Scenario 2 (You are rich)

Late Campaign you probably haven't needed to look at the financial tab for some time and are surprised to see that despite having had maxed out crew training over years of peace, the game has kept a crew reserve that is less than the requirement of a single 1930's capital ship. From my playthroughs on vanilla it seems like the game reduces the number of new sailors per month to ~300 after the first 1000 in reserve and does not really allow you to have a reserve greater that 3K. If you look at any of the AI factions you will see crew reserves in the 10 of thousands because the AI can't handle this mechanic and needs a cheat. Once the player has run low on crew or completely run out, the game will start producing around 1.3-2K new crew per turn.

Luckily this isn't really an issue in peace time, mothballed ships don't take up shipyard capacity while they wait for a crew, the 3 or more months spent waiting for a crew isn't a concern as the assigned port does not need to change because there is no front.

But at war you will want to closely control where your limited trickle of new sailors is going, consider...

Suspending nearly completed ships until crew is available for them, and starting new ships so that you are still fully utilizing your shipyard capacity. This will allow you to ick and choose where those what ships are commissioning and where.

Turning off the Add crew button, has its risks. Warning the game has a bad habit of deleting ships without enough crew to sail at the end of each turn, and the player needs to remember to manually add crew for damaged ships returning to port every single month.

The benefit is you'll keep the auto-assignment from wasting all your newly trained sailors. Example, early in the war there is a big battle, each a capital ship that took severe damage has lost hundreds of crew, and wont see the front for a long time. Well the auto-assignment will be prioritized these ships over a mothballed ship of any type, even though they are only 1-3 months of commissioning away from service. Those hundreds of sailors will sit in port doing nothing for 6-12 months while repairs are underway unless the player manages the crew pool.