comparing 2 worksheets and counting the total number of customers who appear on both

Hi I have 2 worksheets with customer numbers in 'column a' of both worksheets

I want to count the total number of customers who appear on both workseets

e.g. the count would be 2 in the example below
Worksheet 1

Customer number
105
110
115

Worksheet 2
Customer number
100
105
110