Yes, you can just start with a couple GPUs - if you are buying the rest of the mining rig from scratch, however, make sure you get a motherboard with more PCIe slots so that you have the ability to add more in the future. I started with a simple 2-GPU mining rig and just added a third GPU as funds allowed.

Here is a good resource site for choosing what GPUs to buy: http://miningcharts.com

There is a comparison chart here http://miningcharts.com/gpu-mining-comparison-chart/best-price-and-power-usage/ that actually helps calculate purchase and power cost for GPUs over the course of a year. I was going to start very cheap and just get a 1050Ti or two as they are about $100 each, but based on power usage and purchase cost, instead of a few 1050 Ti's I went for a single 1070 Ti. It uses less power, and even costs less per hash to purchase (granted, if you only had $100 to spend you could get a 1050 ti and you would have to save up $400 more to buy a 1070 Ti)