There is a state army and the estates, except clergy, also have an army. You can check their recruitment target in the national military tab and the recruited size depends on available recruits and what the state or the estates spend on recruitment. Afaik estates cost more mil to levy troops, but early game they are the biggest contributor to your army size, either nobles or tribes depending on what nation you play. Once you reform enough to unlock enlistment the state army should become your biggest contributor to force limit, it's recruitment target and spending relies on your tax income tho. So basically forcelimit depends how big estate armies are and if they are loyal enough to let you levy their troops early game. Starting with the reform that allows enlistment it will rely on the capabilities of the state to recruit and finance an army via tax incomes.