There is no limit to the number of options that may be granted as ISOs. However, one of the requirements for ISOs in the Internal Revenue Code is that the fair market value of the options which may be first exercisable in any calendar year cannot exceed $100,000. ISOs over this $100,000 threshold at the time of grant automatically become NQSOs for tax purposes, even if your grant agreement does not state this, but the options under the threshold retain ISO tax status. In addition, at the time of the option grant, you may not own stock with more than 10% of the total combined voting power of all classes of stock of the company.

For how the $100,000 limit is calculated, see another FAQ.