It used to be just 250 for both Lync Online and Lync Server 2010
Microsoft has boosted the maximum number of participants in a Lync Online conference no less than four times what it used to be, according to Anthony Murphy, Microsoft Technical sales for Unified Communications and Cloud Services.“We've increased the supported maximum number of participants in a Lync Online conference from 250 to 1000. We will be updating the documentation over the coming months to reflect this,” he revealed.
When it comes down to audio/video/web conferencing, the meeting attendee capacity used to be just 250 users, but not just for Lync Online, for Lync Server 2010 as well.
Microsoft has yet to specify whether the new maximum number of participants limit in Lync Online is also valid for Lync Server 2010, however, this is bound to happen as the documentation around the unified communications offerings is updated.