Well given the requirements for moods on the wiki, the more dwarfs you have the more likely you are to have one that qualifies for a mood. So limiting the number of dwarfs in a fortress can lead to less moods.
In order for a dwarf to be struck with a strange mood, three conditions must be met:
• There is no currently active strange mood,
• The maximum number of artifacts is not met,
• There are at least 20 eligible dwarves (see below).
If all three of these conditions are true, the game may trigger a strange mood according to the frequency.
Eligibility
The deciding factor for eligibility is a dwarf's actual profession.Thus, dwarves may enter strange moods regardless of what skills they have or don't have, so long as they are of an acceptable profession. Dwarves who have already created an artifact are not eligible to create another, and since every mood ends in either an artifact or death, every dwarf may enter at most one mood. Dwarves who have obtained one or more legendary skills without creating artifacts may enter strange moods.
Appointed nobles may enter moods as per their default profession, but immigrant nobles are not eligible for moods. That includes:
Advisor, Baron/ess, Baron/ess Consort, Count/ess, Countess Consort, Duke/Duchess, Duke/Duchess Consort, Dungeon Master, Hammerer, King/Queen, King/Queen Consort, Philosopher, and Tax Collector.
Further, dwarfs with a military profession other than "Recruit" can not enter moods. Incidental military skills make no difference - eligibility (and weighting) depends purely on the actual profession, as listed at the time. Military professions include:
Axedwarf, Axe Lord, Champion, Crossbowdwarf, Elite Crossbowdwarf, Hammerdwarf, Hammer Lord, Macedwarf, Mace Lord, Marksdwarf, Elite Marksdwarf, Speardwarf, Spearmaster, Swordsdwarf, Swordmaster, Wrestler, and Elite Wrestler.
Babies may not enter moods.
Any other profession is eligible to enter a mood, but not all have the same chance to enter a mood...
So back to my point if you have limited manpower you may not have 20 eligible dwarfs