It depends on what you're trying to to do when it comes to playing MTTs. Are you trying to win or are you fine with just cashing?
A good baseline for ITM is 10% since that usually what most sites pay out (10% of the field). On some sites, payouts could be higher, but an ITM% of 10% or better is decent. If you're a really tight player or play to just cash, your ITM% can be much higher.
As far as ROI% goes, anything positive is always good. I think between 20%-35% is the average of most top MTT players are looking at over a large sample size but I could be wrong.
Again, it would depend on your goal when playing an MTT. If you're looking to just cash, your ROI% could be low (still positive) while your ITM% could be extremely high. If you're playing to win, your ITM% could be much lower (since you're taking more risks to run deep), but your ROI% will be much higher for those times you do run good, make it to the final table, and finish in the top 3 where most of the money is made.
As far as sample size goes, the more MTTs you play, the more accurate your ROI and ITM percentages will be. So, don't necessarily give yourself a set goal and just play as many as you can. As a beginning MTT player, I wouldn't focus on your ITM% and ROI% too much. Instead, focus on your MTT game and constantly improving, and your results will show over time.