In Skype for Business Server you can change the limit from 250 participants to 1000 (I think) per meeting, but that requires that your hardware can handle it. For Skype for Business Online the limit is 250 and can't be changed.
If you are changing the server limit you should probably have a dedicated Front End Pool for Meetings, this should be scaled to handle large meetings. Be aware that the limit is per meeting so you could have 10 meetings with 50 participants, 100 meetings with 5 participants at the same time as you host the meeting with 1000 participants, that is 2000 participants at the same time and will require a lot of CPU and memory on the server.
Also a meeting with more that 250 participants (even more that maybe 50 participants) will probably be a meeting with one or a few presenters and the rest will just listen, then a Skype Meeting Broadcast setup or a Live Event would work better for you.
