Relationship between provincial manpower and lfl (land force limit from now on) - none.
Lfl primarily depends on number of provinces, biggest gain youll notice is from acquiring new provinces.
Secondary lfl bonus comes base tax.
And terciary comes from tax income.
Number created by this 1.) number of provinces 2.) base tax in provinces 3.) tax income, is then multiplied by lfl modifiers (such as national idea, sliders etc) and then you get the final number.
To answer your first question, how to increase the number:
a.) grow bigger (more provinces, regardless of base tax, thats the biggest multiplier on base lfl from what i can tell)
b.) improve provincial base tax - well you cant do this directly, but you can go innovative which will trigger agricultural reforms event that gives +1 base tax more often
c.) get more tax income in your provinces - this one is a bit tricky. Basically there are 2 ways to get them from buildings and from having a center of trade (CoT itself should give a minor tax income, i think it was 2) but for every 100 worth of trade it will add additional +2 tax income, so if you get a wealthy CoT (2000+) your land force limit can get very high, even if you have only a few provinces.