Definitely acceptable as conservative. People only say to avoid patterns as first suits because it's more obvious whe you're wearing it alot. But anything with a suble pattern like this (others include pick & pick, non-shiny sharkskin, nailhead, birdseye, micro-houndstooth, flannels) is almost as conservative as solid.
I always found that annoying - "business casual".
When firms say this, they tend to not mean business casual the way it is discussed on styleforum/malefashionadvice, etc. They usually just mean you don't have to wear a tie, and you should still come if you're broke and don't have a suit. If you go to these events in something truly business casual, you will likely be the only student in a sport coat, odd trousers and double monks in a sea of suits worn sans-tie. The really old partners might not wear their suit jackets and just wear a dress shirt, suit pants, and no tie. Some other old guys will be in conservative sport coats, with pants from a suit. Most of the visiting law students wind up donning a full suit, so that is what you should expect. There'll be a few students who just wear a dress shirt and dress pants, without a jacket. There'll probably be a junior-to-midlevel associate who takes the opportunity to wear a sweet bespoke sport coat that he bought but can't really afford yet...
Nobody will really care what you wear, as long as it isn't track pants or jeans and a t-shirt. If you have a sport coat or blazer you could wear it. Suit without tie if you want to blend in. Suit with tie would be fine, too, as there are inevitably people who are either trying hard or just straight-up like ties (like me). Wear whatever fits within these parameters and makes you feel confident. The most important thing is the conversations you have with people, not how you look. Odds are the receptions will make no difference, but there is the chance that you might hit it off with someone from the firm who makes a note on you, and that can help you chances later with OCIs/In-firms. Nobody will make a note that you dressed perfectly for the occassion, but it can get you bonus points in that it makes you come accross as more mature and confident.
You can get by with two, switching every other day. I would suggest 3. I would not suggest buying more than three, as you likely don't know what suits fit you well yet. Unless you have the $$$$$$ to jump straight into made-to-measure clothing, it takes a lot of time and trial and error to learn about what fits you.
For articling, I would bump it up to 5 or 6. Before buying more suits, think about what fits you and what doesn't, based on articling. Remember that stuff will happen. You don't want to be in a position where you have two suits and one has salad dressing stains on it, but you don't have a backup, and you have to wear the same suit three days in a row because you don't have time to deal with dry cleaning.
As said around 100 times in this thread, don't buy a black suit. If you have one, you can wear it. Nobody will care. But black really is for weddings, funerals, and evening wear.