We don't have many toys because I think an explosion of toys everywhere isn't good for kids. I don't put a number on it, and I'm more interested in the quality of the toys. I don't give any consideration as to whether or not it's "Montessori". The most important thing is does your child actually play with it. If a toy isn't played with or just gets chucked around, get rid of it.
I think if you have something from each of these areas you have plenty...
A construction toy: magnet tiles, duplo, lego, building blocks. Something they can build with.
Something they can look after: a doll or teddy bear that they can roll play nurturing.
Small figurines: can be anything. Peg dolls, duplo or Lego figures, character figures, toy animals. Whatever helps them create small world play.
Toys that move: vehicles of some sort. A ball. Bike. Scooter. This sort of thing.
Fabrics/blankets: for den building
Something crafty: play dough, stickers, paper and crayons.
I'd say if a child has at least one thing from each of these categories then they will be absolutely fine and have tons of play opportunities.
Our home is small and our 3 year old has the following:
Brio train set and trains
Duplo
Magnet tiles
Brio builder set
Basket of vehicles and garage
Little basket on wheels for carting things about the house
Torch
Blankets and big clips for den making
Basket of teddies
Sand table
Mud kitchen
Swing
Bike
Scooter
Books are only limited to what can fit on our bookshelf and we make good use of our library.