I will translate Naomi's answer for you;
"You use 'maar' in combination with a numeral, 'alleen' indicates a limitation. You can also use the words one after another "alleen maar". This has the same meaning as just 'alleen', but with a bit more emphasis.
If you have any questions, feel free to contact me.
'Maar' can also be used to announce an opposition. Example: 'Ik wil wel komen, maar ik mag niet van mijn moeder' of 'Het is een mooi schrift, maar de kwaliteit is slecht'."