The number of people who applied for the course was 120 compared with an initial estimate of between 50 and 100.

estimate 是名词，意思是"估计、估算"，指根据现有信息对数量或价值的推测判断。这个词来自拉丁语 aestimare（评估、估价），在英语中常用来表示对数字或规模的预测性判断。