How much data (numbers) can I store in a database every minute (for 10-20 users)
If I did that would I overload the database and make a lot of d-sync or lag?
There’s no “max-value” as to the obvious limit of MySQL, which depends on the data type. And if you do thing the right way you don’t have to worry about desync or lag, it was made to handle more data than 10-20 users 
Also, why every minute?
Every minute may cause some lag. The default is 10 minutes in ESX. Personally, every 30 minutes would be more than fine. As well as saving it when you log out. MySQL-Async usually tells you in the console how many ms each query is taking, if it’s more than 1000, you probably should do it less frequently.