command to check RAM slots in motherboard?

You can use this command:

sudo lshw -class memory 

It will give you an output like this:

 *-memory
       description: System Memory
       physical id: 33
       slot: System board or motherboard
       size: 4GiB
     *-bank:0
          description: DIMM [empty]
          physical id: 0
          slot: ChannelA-DIMM0
     *-bank:1
          description: DIMM [empty]
          physical id: 1
          slot: ChannelA-DIMM1
     *-bank:2
          description: SODIMM DDR3 Synchronous 1333 MHz (0.8 ns)
          product: AD73I1C1674EV
          vendor: Fujitsu
          physical id: 2
          serial: 43D30100
          slot: ChannelB-DIMM0
          size: 2GiB
          width: 64 bits
          clock: 1333MHz (0.8ns)
     *-bank:3
          description: DIMM [empty]
          physical id: 3
          slot: ChannelB-DIMM1

Here in my system I have 4 memory slots in which currently I am using only one slot. In other slots you can see it shows empty.

For maximum supportable memory use:

sudo dmidecode -t 16

or

sudo dmidecode -t memory

or

 sudo dmidecode | grep -i "Maximum Capacity:" | uniq

EDIT: more interactive way to see Slot use small script (suggested by Serg)

sudo lshw -class memory | awk '/bank/ {count++} END {print "You have " count " slots for RAM"}'

Another way to get all the numbers directly;

To get the total number of slots:

sudo dmidecode -t memory | grep -c '^Memory Device$'

To get the number of slots used:

sudo dmidecode -t memory | grep -c -Po '^\tPart Number: (?!\[Empty\])'

To get the maximum capacity:

sudo dmidecode -t memory | grep -Po '^\tMaximum Capacity: \K.*'

ubuntu@ubuntu ~ % sudo dmidecode -t memory | grep -c '^Memory Device$'
4
ubuntu@ubuntu ~ % sudo dmidecode -t memory | grep -c -Po '^\tPart Number: (?!\[Empty\])'
1
ubuntu@ubuntu ~ % sudo dmidecode -t memory | grep -Po '^\tMaximum Capacity: \K.*'
32 GB