Don't forget to count the number of attendees.

Literal

Not-forget-[without-ず] attendees [possessive-の] member-count [object-を] count-[soft-command-なさい]

員数 (いんずう) is a slightly stiff word meaning 'number of members/units in a count' — compound of 員 (member) + 数 (number). More everyday alternatives are 人数 (number of people) or just 数 alone.