If your goal in the foreseeable future is to fly mainly within Europe for a lo co then your passport won't fill up quickly. I have no idea what the go is for a long haul pilot. You'll probably get visas for the US any way. They take up a page or two depending on which ones you get.
If your goal is to fly for a bizjet operator then your passport will fill up fairly quickly. Most Customs officers generally keep the stamps fairly tidy but some just randomly stamp any thing any where.