How many data points for test set in a time series I have a monthly sales data set from 2018 January onwards. I would like to know from expert what is the optimum train test split and minimum train test split. Also to mention that my data includes 2020 year data where the sales have been influenced due to pandemic, and 2021 have been recovering year.

There is no theoretical optimum for this decision and will be highly problem-dependent.
One option to circumvent this decision is to use an online retraining model. At each point in time, train on previously observed data and test on the next point.
This is illustrated here under the section 'Walk Forward Validation'.
