Setting a value to v-text field from counting a number of documents i have in firebase with vuejs

This is my template where i am trying to put a value automaticly inside of a text box instead of user. I want the value to be the last ID of my documents + 1.

              <v-text-field style="width:170px"
                
                value=""
                label = "ID_Naloge"
                v-model="ID"
                prepend-icon=""
                type = "number"
                :rules="rules.name"
                placeholder=""
                disabled
                
                required
              >
              </v-text-field>

This is my import:

var b = this.countLastItem();
      console.log(b);

This is what i want to return:

ID: parseInt(Number(this.b)) + 1

This is my method that is returnih Nan everytime instead of number eg. 44 (because there are 44 documents + 1):

countLastItem(){
       db.collection("Kontrolne naloge").get().then(function(querySnapshot) {      
        console.log(querySnapshot.size);
        return Number(querySnapshot.size);
    
});

the console log returns 43

this is how my submit code looks like:

submit() {
        this.snackbar = true

       
   

         const project = {
                     ID_Naloge: Number(this.ID),
                     Kriterij_FK: Number(this.Kriterij),
                     Letnik_FK: Number(this.Letnik),
                     Naslov: this.Naslov,
                     Poglavje_FK: Number(this.Poglavje), 
                     Predmet_FK: Number(this.Predmet),
                     Rešitev_FK: Number(this.Rešitev),
                     Stopnja_Težavnosti_FK: Number(this.Stopnja),
                     Točke: Number(this.Točke),
                    
            }
           db.collection('Kontrolne naloge').add(project).then(() => {
              console.log(this.ID)
          })
            this.resetForm();
      },