Your maximum position size depends on your account size:
Account size | Max minis | Max micros |
50K Prime | 4 | 40 |
100K Prime | 6 | 60 |
150K Prime | 10 | 100 |
Remember that 1 mini equals 10 micros. You can mix the two, as long as your total open position never goes above the equivalent of your mini limit. Example on the 50K Prime: 2 minis plus 20 micros equals 4 minis in total, which is exactly your limit.
There is no scaling plan, so these limits are available in full from your very first trading day.