Hey Nikiniki,
The busiest time here is July-August (to be avoided if possible), and there are also a good number of tourists in June and September. It it rather quiet from October to May, but for ex. in Tivat and Kotor you have people living year-round, universities, expats... Budva for ex. gets more empty in the winter.
The best periods to be in Montenegro are May-June, and September-October. Good weather, not too many people.
Cheers,
Carine