This numbers memory game is a math curriculum activity, and it is such a fun way to boost memory, reinforce number recognition and quantity to numeral association, and it allows for movement. A toddler can play alone (like here) or with a sibling or a friend. This activity is very simple too: no specialized Montessori material is required, and it is easy to DIY.
You will need a drawstring bag with numbers (or a box with a lid); a bowl with counters (marbles, beads etc) and a bowl for a child to carry the marbles from another room since counters are placed in a different location.
A child comes back and picks a number (putting numbers in a drawstring bag added a little mystery to it).
A child would say the number outloud.
Then, leaving the numeral behind, the child will take an empty bowl, walk to where the counters are and pick the exact quantity of marbles to bring back to the rug. Since while remembering a numeral he picked, a child has to walk to the other room to get that many counters to bring back to the rug, this activity promotes concentration and memory.
If you have few children, to involve all of them, a variation of this game can be that the child that retrieves the number keeps it a secret and hides it under the rug or turns the number upside down. When this child comes back to the rug with a quantity in basket; the waiting children count the beads/quantity brought and match to the number that was hidden. As a result, children learn counting, numerals and order.
Another extension of this game is incorporating color in the memorization process. A child needs to recall the numeral and the color. You can use colored bead or poker chips and mark the number with a colored dot and then a toddler needs to bring back for example: four blue marbles if number four was color-coded blue. As a result, a child is remembering the numeral and the color.
More on using marbles here.