If you understand gearing and calculations it is easier to determine what range you need and what works bets for your terrain and level for riding.

Gear inches are a measure of this and a range of 40-100 seems to be ideal for most average riders while those who climb more or ride faster will want to expand on that.

A 1 by 7 on an mtb with a 42 tooth ring and a fairly standard 13-28 freewheel will give a 37 to 75 gear inch range so might be lacking for those who want more top end, a cassette will offer an option for an 11 tooth cog which will bump that top gear up to a very useable 95 gear inches.

For most, 75-80 gear inches is as much gearing as you would want to push on the flats for any extended period while gearings above that are nice for descents and tailwinds.

One could always use a larger chain ring and a wider range cassette to expand the gear range.