I think a lot of Roman Catholics are incorrectly taught about the first one. Technically, one isn't praying
to them (they are not deities/to be worshipped!) but praying
through them - however most Roman Catholics don't know/think about the distinction. People use words like "pray to" which then - understandably! - confuses other denominations about what it is we're (meant to be) doing.
The saints and the Virgin Mary have no power in or of themselves - but the understanding according to Sacred Tradition is that these are holy people who reside in Heaven (rather than Purgatory) and who are close to God. As such, we live and pray in the hope that, by petitioning these saints/the Virgin Mary to support us in prayer by taking our prayers to God, these prayers may be more likely to be answered - or that we will become more like these saints/the Virgin Mary, i.e. get better at listening/adhering to God's will.
I liken it to asking a particularly devout aunty/grandparent/member of your church congregation to pray for you: it's the same principle, to my mind.
As for seven sacraments: I don't know how other denominations define sacraments but in Roman Catholicism, a sacrament is a tangible/visible symbol of God's presence in the world. Hence there being so many
There are probably deeper theological reasons than that, but I'm afraid I haven't read up on/thought about this topic much